Strobe Alarm

This strobe gives a visual indication of a sensor input. The input signal causes U1, a light dependent resistor, to charge C1 and C3 through R4. When NE1 fires, C3 discharges into SCR1, which triggers it and causes C2 to discharge through trigger transformer T1, which triggers Flashlamp FL1. The 330-V supply should have about 50 to 100 uF output capacitance. L1 supplies about 25-mH inductance to prolong the flash and the life of FL1.

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Three Dial Combination Lock

Here's an effective little combination lock that you can put together in one evening's time. To open the lock, simply dial in the correct combination on the three rotary or thumbwheel switches. With the correct combination entered, current flows through R1 into Q1's gate terminal, causing the SCR to latch in a conductive state. This sends a current through relay K1, which responds by closing its contacts and actuating whatever load is attached. After opening the lock, twirl the dials of S1 through S3 away from the correct combination so that nobody gets a look at it. The lock will remain open and your load will remain on because the SCR is latched on. To lock things up, it's only necessary to interrupt the flow of anode current through the SCR by pressing pushbutton S4.

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Electronic Combination Lock

When button S12 (#) is pressed, a positive voltage fed through Rl appears at the base of transistor Ql, turning it on. When Ql is conducting, pin 1 of Ul is brought to ground (low) or the battery's negative terminal. With pin 1 low, two things occur: Pin 8 of Ul goes high (+ 9 volts dc), turning on LED 1-indicating that the circuit has been armed-and pin 13 goes from high to low. Transistor Q2 requires a low signal or negative voltage on its base in order to conduct. It also needs a positive voltage on its emitter and a negative voltage on the collector. As long as the door switch (Sl5) remains open (with the door itself closed), Q2's emitter will not receive the necessary positive voltage. If, however, an unauthorized person opens the door, thus closing switch S15 and placing a positive voltage on the emitter of Ql, the following sequence occurs:

1) Transistor Q2 conducts, receiving the necessary biasing current through a current-divider network consisting of resistors R3 and R4.

2) As Q2 conducts, a voltage drop is developed across the voltage dividers made up of resistors R5 and R6. With R5 at 10,000 ohms and R6 at 1000 ohms, approximately one volt appears at the gate of SCR1. That's enough voltage to trigger the SCR's gate.

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Digital Entry Lock

A block pinout diagram of the LS7220 keyless-lock IC is shown. The keypad must provide each key with a contact to a common connection. In this case, the common connection goes to the positive supply rail so that when a key is pressed, a positive voltage is passed through to the wire associated with that key. Each of the 12 keys are brought out to separate wires, and each wire is connected to a different pin of a 24-pin socket (SO1).

To activate (unlock) the circuit, a preprogrammed four-digit access code must be entered in the proper sequence. The four-digit access code must be entered in the proper sequence. The four-digit access is programmed into the circuit by connecting jumpers between terminals of a 24-pin plug-in header.

When the correct access code is entered (in the proper sequence), positive voltages appear at pins 3, 4, 5, and 6 of Ul. That causes Ul to output a positive voltage at pin 13, which is fed through resistor R2 to the base of Ql, causing it to conduct. With Ql conducting, its collector is pulled to ground potential, energizing relay Kl. The normally open relay contacts close, switching on any external device.

Capacitor C2 controls the total time that the output of Ul at pin 13 is positive after the release of the first key. With a value of 3.3 uF for C2, active time after release of the first key is about two seconds, assuming a 6-V supply or four seconds with a 12-V supply. Therefore, if you push the subsequent keys too slowly, the relay might not close at all! To increase the time allotted for code entry, you will have to increase the capacitance of C2.

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Vehicle Security System

This alarm gives a 15-20 second exit and entrance delay. After being triggered, the alarm sounds for five minutes and then shuts off. Once triggered, the sequence is automatic and is not affected by subsequent opening or closing of doors.

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Series/Parallel Loop Alarm

Two SCRs are used with two sensor loops. One loop uses series switches, the other loop parallel switches. When a switch actuation occurs, the SCR triggers. The alarm should be a noninterrupting type.

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Self Latching Light Alarm With Tone Output

A decrease in the resistance of the CDS cell when light strikes it activates latch a and b, enabling tone oscillator c and d which produces an output of about 1000 Hz. RA sets the trip level. S1 resets the circuit.

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Security Alarm

This alarm features open- and closed-loop detector and automatic alarm shutoff. Offers 15 second exit/entrance delay. Alarm on time can be adjusted from 1 to 15 minutes.

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Light Beam Alarm Preamp

This circuit can be used for light beams to 20kHz. The gain of the operational amplifier is set for a 40-dB gain.

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Light Beam Alarm For Intrusion Detection

When the light beam that falls in the CDS photocell is interrupted, transistor (2N3904) conducts thereby triggering SCR1 (C106) and activating alarm bell. S1 resets the SCR. The alarm bell should be a self-interrupting electro-mechanical type.

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Latching Burglar Alarm

When the protective circuit is interrupted (opened), the alarm sounds. To set the circuit, adjust R2 (with protective circuit open) for 1 V across R1.

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Home Security System

This alarm circuit activates when S1 through S5 are activated. This lights LED1 and activates Q1 via IC1C and IC1D. RY1 is wired to self latch. S10 is used to reset. When key switch S1 is activated or when re-entry buttons at S6 are depressed, IC1C is deactivated until RC network R7/C3 charges.

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High Power Alarm Driver

In this circuit, a low-powered SCR is used to trigger a higher powered SCR. When a switch is opening (S2, S3, S4) or closing (S5, S6, S7), either SCR1 or SCR2 triggers. This triggers SCR3 via D1, D2, and R5. BZ1 is a high-powered alarm of the noninterrupting type.

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Door Minder

This circuit monitors a door to determine if it has been left open. After 24 seconds, the alarm sounds. S1 is a magnetic sensor. The alarm is an electronic chime sounds that is struck once per second.

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Differential Voltage Or Current Alarm

The input may be dc or low frequency ac. The output is a distinctive series of audio beeps or a continuous tone, and occurs only when a selected polarity unbalance is present at the input.

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Burglar Alarm With Timed Shutoff

When S1 (sensor) is closed, power is applied to U2, a dual timer. After a time determined by C2, C1 is energized after a predetermined time determined by the value of C5, pin 9 of U2 becomes low, switching off the transistor in the optoisolater, cutting anode current of SCR1 and de-energizing K1. The system is now reset. Notice that (R6xC2) is less than (R7xC5). The on time is approximately given by:

(R7xC5)-(R6xC2)=Ton

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Burglar Alarm

The heart of the circuit is a 555 oscillator/timer, U1, configured for monostable operation. The output of U1 at pin3 is tied to the gate of SCR1. As long as S1 - S5, which are connected to the trigger input of U1, are open, the circuit remains in the ready state, and does not trigger SCR1 into conduction. Because the relay is not energized, battery current is routed through the relay's normally-closed terminal and through current- limiting resistor R3 to LED2, causing it to light.

However, when one of the switches (S1 - S5) is closed, grounding U1 pin2, the output of U1 at pin 3 increases, activating SCR1. That energizes the relay, pulling the wiper of K1 to the normally-open terminal, causing LED1 to light and BZ1 to sound.

The duration of the output is determined by the RC time-constant circuit, formed by R1 and C1. Resistor R2 regulates the output of U1 to a safe value for the gate of SCR1. Switches S1 - S5 are to doors, windows, etc. A switch can be connected in series with B1 to activate and deactivate the alarm circuit when it's not needed.

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Auto Alarm

In operation, the alarm circuit allows a 0 - 47 second time delay, as determined by the R1 /C1 combination, after the switch is armed to allow the vehicle's motion sensor to settle down. This allows you time to get a bag of groceries out of the trunk and not have the hassle of juggling the groceries and the key switch at once.

During the time delay, half of LED1, which is actually a single, bi-colored, three-legged common cathode device, lights green. At the same time, pins 8 and 4 of U2 (a 555 oscillator/timer) are held low by Ul (a 3905 oscillator/timer), causing the alarm to remain silent. Once the delay is over, LED1 turns red, indicating that the circuit is armed.

At that point, a ground at pin 2 of U2 forces pin 3 of U2 high, closing the contacts of Kl and sounding the siren for a time duration determined by R4 and C2. Once the time has elapsed, pin 3 is pulled low, Kl opens, and the circuit is again ready to go. The circuit can be manually reset by the simple expedient of opening and closing the key switch. Potentiometer R3 controls the LED's illumination intensity. Diode D1 ensures that the green segment of LED1 is fully extinguished when Ql is turned on-which turns the LED to red. Resistors R4 and R5 must be connected to the + V bus. not to pin 7 of Ul. otherwise U2 will mysteriously trigger itself each time the initial delay ends.

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