Security Circuits
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The transmitter we recommend to build is quite simple. It can even be built by a beginner. However, unlike the ease of construction and its small size, it is almost certain that it will impress you with its outstanding performance. Its autonomy is 12-15 hours, with a 9 volt battery and its range is from 100 to 300 meters depending on the emission conditions. It's intended for experimentation and amateur use, but it has countless applications... .
This project is an electronic lock with many combinations that you can easily change and opens only with the correct combination of four consecutive numbers. The big advantage of the electronic lock is that the four numbers must be entered in the correct order one after the other. If a number is wrong then the circuit automatically resets to its resting state and the combination of the 4 numbers must go from the beginning. This is important... .
That circuit protects the overdriven signals going into an amplifier. Instead of a zener diode we use a transistor. That way we ensure that the above the ordinary input voltages ( 1 volt r.m.s.) can be acheaved without distortion. Also the use of a transistor limits the number of components used in this schematic.
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 Rl into Ql's gate terminal, causing the SCR to latch in a conductive state. This sends a current through relay Kl, which responds by closing its contacts and actuating whatever load is... .
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 0ow) 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... .
In essence, the circuit is nothing more than an auto ignition coil and a set of points which accomplishes the same thing. A pulsing circuit made from a single CMOS NOR integrated circuit (Ul), opens and closes the relay contacts to simulate the action of the original breaker points. The relay pulser is divided into two clocking functions. The first circuit is a free-running squarewave generator that determines the rate or frequency of the... .
CI and LI resonate on the 1750 meter band, with coverage from 150 kHz to 500 kHz. LI can be slug-tuned for 160-to-190 kHz coverage alone or a 2.5 mH choke can be used for LI, if desired, using CI for tuning. A 1N270 germanium diode rectifies the RF signal and C2 is charged at the peak RF level. This dc level is amplified by an LM358. The gain is determined by R2 and R3, 1 100-kilohm linear potentiometer that varies the dc gain from 1 to 100,... .
The silicon controlled rectifier (SCR) is rated to handle at least the current of the power supply. It is connected in parallel across the 12 V dc output lines, but remains inert until a voltage appears at the gate terminal. This triggering voltage is supplied by the zener diode. At potentials less than 14 V the zener will not conduct current. But, at potentials greater than 14 Vdc the zener conducts and creates a voltage drop across the 330... .
The circuit consists of a multivibrator (Ql & Q2), and a low power output stage Q3. The speaker should have an impedance in the region of 40 to 80 ohms. To use a low impedance speaker, connect an output transformer from the emitter of Q3 to ground. Cl and C2 determine the pitch of the siren and the values specified will provide a tone of about 300 Hz. Quiescent current is negligible. .
The push button and potentiometer initiate a time delay that turns a light on then automatically turns it off again after a predetermined time. The potentiometer can be set for a delay of a few seconds to just under three minutes. When the push-button switch SW2 is pressed, capacitor Cl gets charged through D5 to the full dc voltage developed by the diode bridge. When the button is released, the charged capacitor is connected across the series... .
This circuit checks the reliability of appliances so that the equipment may be used safely. The test circuit must be plugged into a properly wired three terminal wall outlet. When a two-lead or three-lead appliance is plugged into circuit outlet SOI, neon lamps NE1 and NE2 will light if the appliance is safe .
The single-chip, burglar -alarm circuit shown uses a dual 556 timer, draws 10 mA of standby current, and generates a pulsing alarm signal that conserves battery energy. Once activated, the alarm wilLremain on, independent of the subsequent state of any of the sensors. The sensors support both deferred and immediate-action modes. You can attach this circuit to your car"s internal lighting circuitry using a single wire and a relay. To arm... .
False alarms produced by semiconductor failure are impossible with this burglar-alarm circuit equipped with relays. What"s more, the circuit is virtually immune to false triggering. With a standby current of less than 0.1 mA, the circuit offers all the features an alarm needs: entry and exit delays, a timed alarm period, and automatic reset after an intrusion. One CMOS CD4093B quad NAND gate, IC1, supplies both logic and analog timing... .
The heart of the circuit is a 555 oscillator /timer, Ul, configured for monostable operation. The output of Ul at pin 3 is tied to the gate of SCRl. As long as Sl-S5, which are connected to the trigger input of Ul, are open, the circuit remains in the ready state, and does not trigger SCRl into conduction. Because the relay is not energized, battery current is routed through the relay"s normally-closed terminal and through current-limiting... .
The tracer, consisting of both a transmitter and receiver, is designed to follow a closed-loop wire or cable system. It follows an induced voltage path created by feeding a low-current, audio-frequency signal through the cable. When the pick-up coils come within close proximity of the current -carrying cable, a small voltage is generated in each coil, and that induced voltage is then processed by the receiver"s circuitry. .
The bug tracer is made up of a simple rf-injector circuit consisting of Ql and Q2, and a pocketsize, AM broadcast receiver. The two-transistor rf-injector circuit supplies a constant rf signal to one end of a cable. Then the AM receiver is used as a detector, allowing you to trace the wire to its source. Transistor Ql, along with piezoelectric ceramic resonator RESl, make up a simple rf oscillator that operates either at or near the AM-radio,... .
Industrial applications sometimes require that power be only briefly applied to a load following the closure of a switch, such as a microswitch or foot switch. The load could be a heating element for use in sealing plastic bags, a de motor that is indexed or stepped with each application of power, or a de solenoid which is to be energized for a brief time-for example, a staple gun. .
The spark developed by the circuit is suitable for a gas ignitor. Capacitor Cl is charged through Rl and Dl toward peak line voltage. C2 is simultaneously being charged at a slower rate through R5. When the charge on C2 is sufficient.to trigger the PUT, the SCR is triggered on, providing a rapid discharge path for Cl through the transformer primary. The SCR is triggered about 20 times per second with the component values shown. .
This rf detector can locate low-power transmitters (bugs) that are hidden from sight. It can sense the presence of a 1-mW transmitter at 20 feet. which is sensitive enough to detect the tiniest bug. As you bring the rf detector closer to the bug, more and more segments of its LED bar-graph display light, which aids in direction finding. The front end has a two-stage wideband rf amplifier, and a forward-biased hot-carrier diode for a... .
The door opener derives its power from a 9-V battery. A momentary-contact switch, 52, is provided in the event that manual opening and closing is required. Relay Kl is a 9-V type and relay K2 is a 117"Vac latching-type, which automatically latches with the first burst of current and opens on the second burst. The gate lead of the LASCR is not used; a light source triggers the LASCR unit into conduction, causing current to flow in the... .
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