"Develop Your Creations"

get a lot of electronic circuits

"Develop Your Creations"

get a lot of electronic circuits

"Develop Your Creations"

get a lot of electronic circuits

Intelligent Electronic Lock

This intelligent electronic lock circuit is built using transistors only. To open this electronic lock, one has to press tactile switches S1 through S4 sequentially. For deception you may annotate these switches with different numbers on the control panel/keypad. For example, if you want to use ten switches on the keypad marked ‘0’ through ‘9’, use any four arbitrary numbers out of these for switches S1 through S4, and the remaining six numbers may be annotated on the leftover six switches, which may be wired in parallel to disable switch S6 (shown in the figure). When four password digits in ‘0’ through ‘9’ are mixed with the remaining six digits connected across disable switch terminals, energisation of relay RL1 by unauthorised person is prevented.

Intelligent Electronic Lock circuit diagramFor authorised persons, a 4-digit password number is easy to remember. To energise relay RL1, one has to press switches S1 through S4 sequentially within six seconds, making sure that each of the switch is kept depressed for a duration of 0.75 second to 1.25 seconds. The relay will not operate if ‘on’ time duration of each tactile switch (S1 through S4) is less than 0.75 second or more than 1.25 seconds. This would amount to rejection of the code. A special feature of this circuit is that pressing of any switch wired across disable switch (S6) will lead to disabling of the whole electronic lock circuit for about one minute.

Even if one enters the correct 4-digit password number within one minute after a ‘disable’ operation, relay RL1 won’t get energised. So if any unauthorised person keeps trying different permutations of numbers in quick successions for energisation of relay RL1, he is not likely to succeed. To that extent, this electronic lock circuit is fool-proof. This electronic lock circuit comprises disabling, sequential switching, and relay latch-up sections. The disabling section comprises zener diode ZD5 and transistors T1 and T2. Its function is to cut off positive supply to sequential switching and relay latch-up sections for one minute when disable switch S6 (or any other switch shunted across its terminal) is momentarily pressed.

During idle state, capacitor C1 is in discharged condition and the voltage across it is less than 4.7 volts. Thus zener diode ZD5 and transistor T1 are in non-conduction state. As a result, the collector voltage of transistor T1 is sufficiently high to forward bias transistor T2. Consequently, +12V is extended to sequential switching and relay latch-up sections. When disable switch is momentarily depressed, capacitor C1 charges up through resistor R1 and the voltage available across C1 becomes greater than 4.7 volts. Thus zener diode ZD5 and transistor T1 start conducting and the collector voltage of transistor T1 is pulled low. As a result, transistor T2 stops conducting and thus cuts off positive supply voltage to sequential switching and relay latch-up sections.

Thereafter, capacitor C1 starts discharging slowly through zener diode D1 and transistor T1. It takes approximately one minute to discharge to a sufficiently low level to cut-off transistor T1, and switch on transistor T2, for resuming supply to sequential switching and relay latch-up sections; and until then the circuit does not accept any code. The sequential switching section comprises transistors T3 through T5, zener diodes ZD1 through ZD3, tactile switches S1 through S4, and timing capacitors C2 through C4. In this three-stage electronic switch, the three transistors are connected in series to extend positive voltage available at the emitter of transistor T2 to the relay latch-up circuit for energising relay RL1.

When tactile switches S1 through S3 are activated, timing capacitors C2, C3, and C4 are charged through resistors R3, R5, and R7, respectively. Timing capacitor C2 is discharged through resistor R4, zener diode ZD1, and transistor T3; timing capacitor C3 through resistor R6, zener diode ZD2, and transistor T4; and timing capacitor C4 through zener diode ZD3 and transistor T5 only. The individual timing capacitors are chosen in such a way that the time taken to discharge capacitor C2 below 4.7 volts is 6 seconds, 3 seconds for C3, and 1.5 seconds for C4. Thus while activating tactile switches S1 through S3 sequentially, transistor T3 will be in conduction for 6 seconds, transistor T4 for 3 seconds, and transistor T5 for 1.5 seconds.

The positive voltage from the emitter of transistor T2 is extended to tactile switch S4 only for 1.5 seconds. Thus one has to activate S4 tactile switch within 1.5 seconds to energise relay RL1. The minimum time required to keep switch S4 depressed is around 1 second. For sequential switching transistors T3 through T5, the minimum time for which the corresponding switches (S1 through S3) are to be kept depressed is 0.75 seconds to 1.25 seconds. If one operates these switches for less than 0.75 seconds, timing capacitors C2 through C4 may not get charged sufficiently. As a consequence, these capacitors will discharge earlier and any one of transistors T3 through T5 may fail to conduct before activating tactile switch S4.

Thus sequential switching of the three transistors will not be achieved and hence it will not be possible to energise relay RL1 in such a situation. A similar situation arises if one keeps each of the mentioned tactile switches de-pressed for more than 1.5 seconds. When the total time taken to activate switches S1 through S4 is greater than six seconds, transistor T3 stops conducting due to time lapse. Sequential switching is thus not achieved and it is not possible to energise relay RL1. The latch-up relay circuit is built around transistors T6 through T8, zener diode ZD4, and capacitor C5. In idle state, with relay RL1 in de-energised condition, capacitor C5 is in discharged condition and zener diode ZD4 and transistors T7, T8, and T6 in non-conduction state.

However, on correct operation of sequential switches S1 through S4, capacitor C5 is charged through resistor R9 and the voltage across it rises above 4.7 volts. Now zener diode ZD4 as well as transistors T7, T8, and T6 start conducting and relay RL1 is energised. Due to conduction of transistor T6, capacitor C5 remains in charged condition and the relay is in continuously energised condition. Now if you activate reset switch S5 momentarily, capacitor C5 is immediately discharged through resistor R8 and the voltage across it falls below 4.7 volts. Thus zener diode ZD4 and transistors T7, T8, and T6 stop conducting again and relay RL1 de-energises.


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Luxurious Toilet Bathroom Facility

Aged persons in the house and guests often fumble while searching for the toilet and bathroom switches at night. Also, very few of us take care to switch off the lights of toilets/bathrooms after using them. The circuit given here helps to overcome both the problems. The figure shows two symmetrical circuits (one each for toilet and bathroom) sharing common power supply and a melody generator-cum-audio warning unit. The reed switches S1 and S2 are of normally-open type, operated by permanent magnets appropriately fixed to the doors of bathroom and toilet, respectively. When the doors of bathroom and toilet are closed, the reed switches are also closed, and vice versa. (Door is assumed in closed condition with nobody inside bathroom/toilet, i.e. reed switch is activated.)

Luxurious Toilet Bathroom Facility circuit diagram
The operational features of the circuit are:
  • Lamp and exhaust fan are switched on when the door is opened.
  • Soft music is played continuously until the door is closed from inside/out side.
  • With a person inside the room, lamp and fan remain on, until the door is reopened. They go off when the door is reopened.
  • Visual indication of whether the toilet/bathroom is occupied/vacant is given by two bicolour LEDs fixed on a panel, which may be fitted near the door with corresponding ‘toilet’/’bathroom’ labels on them. Here the LED colour turns from ‘green’ to ‘red’ if the room gets occupied, and vice-versa.
  • If the door is opened once, and not closed back within 10 seconds, the lamp and fan are automatically switched off, thus conserving electricity. But the music remains on as a reminder that the door is not closed.
  • For cleaning of bathroom/toilet with doors kept open, a parallel on/off switch is included on the switchboard to bypass the relay contacts and manually control the switching on/off of the light and exhaust fan. (This is the service mode.) In this case, the music remains on as long as the door remains open. In case of failure of the unit, the same on/off switch can be used as usual until the circuit is repaired.
  • Due to battery backup facility, even with power failure, when a person is inside, the door status is maintained. However, the lamp and fan will be on only on mains resumption.
  • Also, when a person leaves the room during power failure, with door closed, the lamp and fan are kept off on resumption of power. (Intelligent-mode!)
  • However, the circuit can be fooled by opening and closing the door within 10 seconds, without entering inside. In this case, the lamp and fan will continue to be on and would require reopening and closing of the door to bring the circuit to order.
This problem can be prevented to some extent by using a hydraulic door opener, which would approximately take 10 seconds to close the opened door. A delay period of 10 seconds is deliberately chosen for letting the person inside the toilet/bathroom in normal case! IC1 is a dual positive edge-triggered ‘D’ type flip-flop. IC1(a) gets triggered when bathroom door (and switch S1) is opened and hence IC1(b) toggles, as Q output of IC1(a) is connected to clock input pin of IC1(b). As a result, relay RL1 energises through transistor T3, thereby switching on the lamp and exhaust fan. (Please refer to Fig. 2, the separate wiring diagram of lamp and exhaust fan via the N/O contacts of the relay.)

Luxurious Toilet Bathroom Facility
Simultaneously, pin 2 (Q) of IC1(a) goes low, switching transistor T5 ‘on’, which switches on melody generator IC4, letting out a sweet audio tune via transistor T6 and loudspeaker. In normal condition, when someone opens the bathroom door and gets inside within preset time of IC3(a) (10 seconds here), and closes the door from inside, the music stops with lamp and fan ‘on’. Now, in case someone opens the door before or after use, and forgets to shut it, the lamp and exhaust fan are switched off after 10 seconds but the music remains ‘on’ as a reminder that the door is to be closed.

This happens due to mono multivibrator (MMV) IC3(a), which resets pin 10 of IC1(b) through transistor T1 after 10 seconds. (This period can be adjusted by varying the values of resistor R11 and/or capacitor C7.) It should be noted here that although IC3 is used as ‘MMV’, it is triggered here with a positive pulse through its pin 4 (reset pin) rather than its pin 6 (trigger pin). This arrangement makes it unique for setting and resetting IC3 through pin 4, and resetting IC1(a) through pin 5 of IC3 and transistor T1. Battery backup facility ensures memory backup during power failure. Power supply uses a normal 2-diode full-wave rectifier circuit, which needs no further explanation.

The purpose of using bi-color LED1 and LED2 is that, initially when the door is closed these emit green light— as the green LED part gets the supply via resistor R15— to indicate that bathroom/toilet is vacant. When bathroom/toilet is occupied, transistor T3/T4 conduct to light up the red LED part as well. Melody generator IC4 (UM66) is switched on through diodes D3/D4 and transistor T5, which conducts when IC1(a) pin 2 or IC2(a) pin 2 goes low. When transistor T5 conducts, zener ZD1 breaks down and supplies regulated 3.9V to IC4, to produce a melodious tune via transistor T6 and the speaker. As most toilets and bathrooms are ‘attached’ nowadays, only a single circuit is required, and the circuit can be wired on a general-purpose veroboard. A small modification of the circuit, by adding additional SPST switch S3, as shown in Fig. 2, needs to be done inside the wooden switchboard box. This permits the user to operate the lamp and fan during cleaning of the toilet or for bypassing the circuit, when bathroom or toilet undergo repair work.


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Automatic Heat Detector

This circuit uses a complementary pair comprising npn metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1. The second part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are connected to the +3V supply when the relay is in energized state, whereas pin 2 is grounded.

Circuit diagram:Automatic Heat Detector circuit diagram
A resistor (R2) connected across pins 7 and 8 is used to fix the frequency of the inbuilt oscillator. The output is available from pin 3. Two transistors BC147 (T3) and BEL187 (T4) are connected in Darlington configuration to amplify the sound from UM3561. Resistor R4 in series with a 3V zener is used to provide the 3V supply to UM3561 when the relay is in energized state. LED1, connected in series with 68-ohm resistor R1 across resistor R4, glows when the siren is on. To test the working of the circuit, bring a burning matchstick close to transistor T1 (BC109), which causes the resistance of its emitter-collector junction to go low due to a rise in temperature and it starts conducting.

Automatic Heat Detector
Simultaneously, transistor T2 also conducts because its base is connected to the collector of transistor T1. As a result, relay RL1 energises and switches on the siren circuit to produce loud sound of a firebrigade siren. Note: We have added a table to enable readers to obtain all possible sound effects by returning pins 1 and 2 as suggested in the table.




DIY Electronics Projects and Circuit Diagrams, Schematics only at www.extremecircuits.net

Musical Touch Bell

Here is a musical call bell that can be operated by just bridging the gap between the touchplates with one’s fingertips. Thus there is no need for a mechanical ‘on’/‘off’ switch because the touch-plates act as a switch. Other features include low cost and low power consumption. The bell can work on 1.5V or 3V, using one or two pencil cells, and can be used in homes and offices. Two transistors are used for sensing the finger touch and switching on a melody IC. Transistor BC148 is npn type while transistor BC558 is pnp type. The emitter of transistor BC148 is shorted to the ground, while that of transistor BC558 is connected to the positive terminal.

Musical Touch Bell circuit diagram
The collector of transistor BC148 is connected to the base of BC558. The base of BC148 is connected to the washer (as shown in the figure). The collector of BC558 is connected to pin 2 of musical IC UM66, and pin 3 of IC UM66 is shorted to the ground. The output from pin 1 is connected to a transistor amplifier comprising BEL187 transistor for feeding the loudspeaker. One end of 2.2-mega-ohm resistor R1 is connected to the positive rail and the other to a screw (as shown in the figure). The complete circuit is connected to a single pencil cell of 1.5V. When the touch-plate gap is bridged with a finger, the emitter-collector junction of transistor BC148 starts conducting.

Simultaneously, the emitter-baser junction of transistor BC558 also starts conducting. As a result, the collector of transistor BC558 is pulled towards the positive rail, which thus activates melody generator IC1 (UM66). The output of IC1 is amplified by transistor BEL187 and fed to the speaker. So we hear a musical note just by touching the touch points. The washer’s inner diameter should be 1 to 2 mm greater than that of the screwhead. The washer could be fixed in the position by using an adhesive, while the screw can be easily driven in a wooden piece used for mounting the touch-plate. The use of brass washer and screw is recommended for easy solderability.


DIY Electronics Projects and Circuit Diagrams, Schematics only at www.extremecircuits.net

High Power Bicycle Horn

An interesting circuit of a bicycle horn based on a popular, low cost telecom ringer chip is described here. This circuit can be powered using the bicycle dynamo supply and does not require batteries, which need to be replaced frequently. The section comprising diodes (D1 and D2) and capacitors (C1 and C2) forms a half-wave voltage-doubler circuit. The output of the voltage doubler is fed to capacitor C3 via resistor R1. The maximum DC supply that can be applied to the input terminals of IC1 is 28V. Therefore zener diode ZD1 is added to the circuit for protection and voltage regulation. The remainder of the circuit is the tone generator based on IC1 (KA2411).

The dual-tone output signal from pin 8 of IC1 is fed to the primary of transformer X1 (same as used in transistor radios) via capacitor C6. The secondary of X1 is connected to a loudspeaker directly. In case you are interested in connecting a piezoceramic element in place of the loudspeaker, remove capacitor C6, transformer X1, and the loudspeaker. Connect one end of the piezoceramic disk to pin 5 of IC1 and the other end to pin 8 of IC1 through a 1/4W, 1-kiloohm resistor. IC1 KA2411 is also available in COB style, with the same pin configuration. Both packages work equally well.

However, to get the best results with the COB package, change values of resistors R2 through R4 to 330-kilo-ohm, capacitor C4 to 0.47µF, 63V electrolytic (positive end to pin 3 of IC1), and C5 to 0.005µF, 63V. This bicycle horn project can also be used as a telephone extra ringer by just removing all components on the left side of capacitor C3 and connecting the circuit shown in Fig. 2 to the terminals of capacitor C3.


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Overheat Detector Alarm/Switch

At the heart of this circuit is a precision integrated temperature sensor type LM35 (IC1), which provides an accurately linear and directly proportional output in mV, over the zero to +155 degrees C temperature range. The LM35 develops an output voltage of 10 mV/K change in measured temperature. Designed to draw a minimal current of its own, the LM35 has very low self heating in still air. Here the output of the LM35 is applied to the non-inverting input of a comparator wired around a CA3130 opamp (IC2). A voltage divider network R3-P1 sets the threshold voltage, at the inverting input of the opamp. The threshold voltage determines the adjustable temperature trip level at which the circuit is activated.

Circuit diagram:
Overheat Detector Alarm/Switch circuit diagram
Overheat Detector Alarm/Switch Circuit Diagram

When the measured temperature exceeds the user-defined level, the comparator pulls its output High to approx. 2.2 V causing transistor T1 to be forward biased instantly. T2 is also switched on, supplying the oscillator circuit around IC3 with sufficient voltage to start working. The 555 set up in astable mode directly drives active piezoelectric buzzer Bz1 to raise a loud alert. Components R7, R8 and C4 determine the on/off rhythm of the sounder. A transistor based relay driver may be driven off the emitter of T1 (TP1). Similarly, replacing the piezo sounder with a suitable relay allows switching of high-power flashers, sirens or horns working on the AC mains supply.

Author: T. K. Hareendran - Copyright: Elektor Electronics 2007
 
 
DIY Electronics Projects and Circuit Diagrams, Schematics only at www.extremecircuits.net

Test Beeper For Your Stereo

The test beeper generates a sinusoidal signal with a frequency of 1,000 Hz, a common test frequency for audio amplifiers. It consists of a classical Wien-Bridge oscillator (also known as a Wien-Robinson oscillator). The network that determines the frequency consists here of a series connection of a resistor and capacitor (R1/C1) and a parallel connection (R2/C2), where the values of the resistors and capacitors are equal to each other. This network behaves, at the oscillator frequency (1 kHz in this case), as two pure resistors. The opamp (IC1) ensures that the attenuation of the network (3 times) is compensated for.

In principle a gain of 3 times should have been sufficient to sustain the oscillation, but that is in theory. Because of tolerances in the values, the amplification needs to be (automatically) adjusted. Instead of an intelligent amplitude controller we chose for a somewhat simpler solution. With P1, R3 and R4 you can adjust the gain to the point that oscillation takes place. The range of P1 (±10%) is large enough the cover the tolerance range. To sustain the oscillation, a gain of slightly more than 3 times is required, which would, however, cause the amplifier to clip (the ‘round-trip’ signal becomes increasingly larger, after all).

Circuit diagram:
Test Beeper For Your Stereo circuit diagram
Test Beeper Circuit Diagram

To prevent this from happening, a resistor in series with two anti-parallel diodes (D1 and D2) are connected in parallel with the feedback (P1 and R3). If the voltage increases to the point that the threshold voltage of the diodes is exceeded, then these will slowly start to conduct. The consequence of this is that the total resistance of the feedback is reduced and with that also the amplitude of the signal. So D1 and D2 provide a stabilizing function. The distortion of this simple oscillator, after adjustment of P1 and an output voltage of 100 mV (P2 to maximum) is around 0,1%. You can adjust the amplitude of the output signal with P2 as required for the application. The circuit is powered from a 9-V battery. Because of the low current consumption of only 2 mA the circuit will provide many hours of service.

Author: Ton Giesberts - Copyright: Elektor Electronics 2007
 
 
DIY Electronics Projects and Circuit Diagrams, Schematics only at www.extremecircuits.net