3 x npn small signal transistors are used, any type can be used here, however varying gains of different transistors, will determine the overall gain of the circuit, the type used here are bc337 same as the LedLabs transistors. Some small transistors like bc547 can give 8 million gain if wired this way as their gain is 200, so 200x200x200. (however this voltage gain never exceeds the power supply voltage in this type of circuit). It means with micro volts as input you can get close to the supply voltage out) This field detector can detect static electric, or even mains, so when there is thunder and lightning in the area this could well be set off. Or it could be used for a ghost detector as electric fields are supposed to change. Connecting the final output to the Monostable on LedLabs will give a visual and audible alert when these fields change. This circuit shows how to wire the three transistors and resistors and link to monostable. Also a 50hz filter can be added to rule out mains if using for paranormal research.
choosing a value for c, say 100nf, gives around 26-27k for the resistor, as we have chosen the cut-off frequency for fc of 60hz we can find a value for r, by rearranging the equation, this is how we found 26-27k for the resistor. this will allow frequencies of above 60hz to pass, and attenuate lower signals like mains frequency.
Sometimes it is easier to choose a value for c because if we left the result for the value of c, it may be some strange hard to find value, it is easier to make or find resistor values. Furthering this circuit more, the output from the monostable could be fed into the 4026 decade counter, then this would respond to the monostable trigger and put a count of 1 on it. (to test this, on some of the earlier LedLabs board without an output trigger connection, just hold a wire on pin 3 of the 555 timer and push the other end into the clock in connection) If a circuit with the emf detector and a monostable and counter was built this could be left in a room and the result stored on the counter, so the observer would know of a trigger pulse while not present. This is a good use of a monostable here as it stops several pulses happening on the counter, because the counter has a clean trigger pulse, from the monostable sometimes called a one shot.