Calibration

To undertake initial setting-up, use a test lead terminated at each end with a crocodile clip. Attach one end to the sensor bolt and the other to a piece of tin-foil about 300mm square. Due to the sensitivity of the circuti, it is important that both ends of the test lead should have a sure connection. Turn carbon preset VR4 back completely. Turn multiturn preset VR3 to 40 kilohms (40k) and multiturn preset VR2 to its maximum setting 50k. Turn the multiturn potentiometer VR1 to its mid-point. If at any time the circuit does not behave as described, switch off immediately and check the wiring carefully. 

Be aware that the presence of your body may affect the turning. Use a plastic or insulated screwdriver to turn VR2 and stand back from the circuit to see weather the crackling stops. If not, continue to back-off VR2 very carefully until the crackle just stops when stand back. Now set switch S1 to activate. The unit should now react when your hand approaches the sensor plate from a distance of few centimeters. Experiment a little to discover the best setting for preset VR2 and potentiometer VR1. A single "crackle" triggers monostable IC2b. 

Note that that the Activate position of S1 is optimised both for small sensors such as the 300mm square tin-foil sensors used in testing and a moderate temperature range (10 degree circus or 15 degree circus variation). Other application may require calibration in the Activate setting, monitoring the voltage across capacitor C10 by means of the test bolts provided.