A touch activated relay driver for automotive use.


The following circuit and information is provided without any guarantee that it is reliable or suitable for any particular purpose. It may contain errors or omissions and it may not work as described.

My old 81 Crown did not have any in theft protection except for a well worn key. I had fitted a hidden switch that de-activated the computer power used for the injectors but wanted something extra.

I was asked if I could make a touch activated device that could be used to de-activate the starter. The idea is that after touching some particular bit of metal you then have about 20 seconds to crank the starter. Deactivating the starter is not a very secure way of protecting a car from a determined thief but there are some advantages to this device. You can not forget to turn it on and there is no switch to find. If you install this type of device a switch under the bonnet or other method should be available to bypass it in the case of a fault or if you give the car in for service etc.

This circuit is based on but not the same as a more complicated one that gave good service over 5 years and was sold with my old car. This new circuit has not had much of a test so only reproduce it if you have the knowledge to trouble shoot any problems and make any design changes that may be necessary.

Home made devices get little real testing and their reliability is such that they should not be used where they could cause a running engine to cut out. Engine cut out devices are also banned in many countries.


R7 D3 and C2 provide a stable supply for the circuit. T2 is held on by R9 -R13 until the touch point is grounded with a resistance less than about 20 M Ohm. R6 D2 C3 R5 combine to prevent the gate of T2 from being destroyed by static electricity. When the touch point is grounded T2 turns off and C1 is charged through R8 and D1. C1 then discharges through R2-R3-R4. The positive voltage on C1 keeps T1 turned on for about 30 seconds. R1 limits the current through T1 and gives it some chance if if the relay wire is momentarily shorted to + 12V. It could be reduced to drive bigger loads. The circuit draws about .005 amps so its battery drain in a week is about the same as having the parking lights on for 6 minutes.


I Made up a small circuit board that would fit in an available box. A .pdf file is provided. CLICK HERE FOR TGPCB.PDF For some information on photo etching a board see CLICK HERE FOR BOARD MAKING

When you print it out make sure that you have all the options in the Acrobat software that alter the print size, turned off. The result should be 2 inches by 1.3 inches. Most holes are .8mm except the diodes and transistors 1.0mm. Make the clearance holes for the wires to just fit. Most of the parts are available in any electronics shop but I had to go to Dick Smith for some positive resist PCB and a 63 Volt 1.5 uF MKT (C1) that was the correct size (w x h x t 7.5 x 12 x 6 mm).

Make sure that if the FETs are pushed over to fit in the box that they are pushed the correct way so they do not short out.

To make the label I used the Open Office drawing function, printed on some good coated paper then laminated both sides with a stick on type laminating film. The back side was laminated so the glue does not damage the label. A thick black border makes it easy to get a good edge finish by cutting down the centre of the line as on a black box cutting imperfections do not show up. If you want to give it a try you can print out this .PDF CLICK HERE


This latest design has not been put in my car but with the original one I connected the touch wire to a screw that was in holding the plastic dashboard together and was thus insulated. The touch point was easy to reach by putting one left hand finger in a shallow hole , touching the metal key in the steering lock completed the circuit. My car started easily so there was plenty of time available but if there is a problem just touching again gives you another 30 seconds. On a really dry day a finger may need some moistening. If you are wearing gloves to stop finger prints you are out of luck. As a matter of fact I miss not having it on my new car as I often leave my wife sitting in the passenger seat when I duck into the shops and to keep the radio on for her requires the key in the lock --what happened to accessory keys-- A key in a lock is an invitation that is getting harder for thieves resist now that cars have immobilisers.