The specific circuit is able to operate a 12 volt relay at the touch of your hand. It works by exploiting the skin conductivity. The circuit is quite inexpensive and simple, and it is based on the classic 555 timer, used at monostable configuration.
By connecting it to a door's handle, it may used as an adequate and effective economic protection for your lab, your room, or even your drawers. By tapping the handle, the intruder will be revealed by the sound of the siren connected to the relay, which will force him to give up the curiosity. With the same type of connection, it will be just as perfect to surprise the host of the party with a swirl of lights connected to the relay. You can also use it for playing back a prerecorded warning-message.
A more funny idea is to be used as a simple anti-theft protection, for the target preferred by "nocturnal predators," the refrigerator. As soon as anyone touches the handle, a small siren will make him to give up from "eating a snack." With all this, we are certain that you will find many more occasions and excuses to use it.
Someone might think that the circuit presented here is somehow "trivial", but certainly can not be said that lacks of originality. We believe, that this is a kind of a suitable project, to stimulate the interest of those who have not tried to build something with their hands, involving them in an area that is part of the life of each of us.
The electronic schematic
To operate the circuit you will need a classic 9 volt battery or a 12 volt power supply unit. In this project the 555 timer is used in a monostable configuration, thus its output (pin 3) remains inactive (at logic 0) until the voltage on pin 2 drops to less of 1/3 of the supply voltage. At that point the timer goes at its active state (turns on) for a time T, that is given by:
T = 1.1 x (R2+R3) x C2
Voltage on pin 2 drops to zero, if someone touches the wire connected to R1 trimmer. This is due to the connection established to earth through the body, which gradually discharges C2 through R1. Consequently, a voltage pulse is formed on pin 3, for time T. Pin 3 drives the base of a transistor, which in turn, energizes the coil of the relay.
A diode LED (DL1) is connected in parallel to the coil of the relay, so that it is turned on when the relay is turned on. Diode DS1 is also connected in parallel to the relay's coil, and it is used to protect the transistor from the inverse voltage induced during relay's transitions.
How to build it
Begin placing the components on the Printed Circuit Board, starting from the 555. Instead of soldering the 555 directly on the board, it is better to use a suitable 8-pin ic-socket. Continue with the resistors and then with the two trimmers. Then, mount the capacitors, taking care to place electrolytic capacitors at their correct polarity.
Continue the assembly by placing DS1 at correct polarity. Remember, that the band which surrounds one side of his body, is closer to the cathode. Then, place and solder the LED, whose longest terminal is the anode.
Normally, when mounting the relay, you will not encounter any difficulty. Just remember to use a relay having its footprint exactly matched to the PCB holes. When mounting the transistor TR1, keep in mind that the small metallic edge of his body, marks the emitter. At this point, you must also mount two terminal blocks; A two-pins terminal block for the power supply, and a three-pins terminal for the output of the three-pole relay.
Adjusting trimmer pots
Turn the screws of both trimmers on one side and then in the other, and then adjust them in their mid-positions. Power on the circuit from a 9 Volt battery or from a 12V power supply unit.
As long as you do not touch the wire connected to R1 trimmer, nothing happens, but as soon as it is touched, the relay is activated and the LED diode lights up. After a certain time, adjustable at will through the trimmer R2, the relay returns to idle.
As we have already explained, the R1 trimmer adjusts the sensitivity of the circuit. If not suitable adjusted, you may notice that the relay will always remain on. If something like that happens, just turn the trimmer until the circuit stabilizes.
Printed Circuit Board artwork and components placement guide.
List of components
R1=1Meg trimmer pot
R2=220K trimmer pot
R3=10K, 1/4W, 5%
R4, R5=1K, 1/4W, 5%
T1=BC107 general purpose NPN or equivalent
Relay=12V DPDT Relay