# Finger/Touch contact circuit question

Discussion in 'General Electronics Chat' started by burtS, Jul 8, 2009.

1. ### burtS Thread Starter Member

Mar 6, 2008
29
0
I posted this some days ago on Projects but had no responses: Trying again on this series:

I'm currently using the attached circuit to drive a relay when finger contact is made between points A and B. Maximum current across contacts must be less than 1 ua (assuming very low finger resistance or short across A and B). The circuit as shown works but I have several questions:]

1. What is the effect of inc. or decreasing relay coil resistance on battery drain?

2. Are R2 and R3 optimal values?

3. MOST IMPORTANT ISSUE: Can I use a NPN Darlington to replace the two 2N2222 transistors? If so, which Darlington would you suggest and would this affect choice of the R2 and R3 resistors?
Many Thanks,
BurtS

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2. ### steinar96 Active Member

Apr 18, 2009
239
4
If the resistance of the coil decreases the current drain should be higher. When the transistor completing the coil-battery circuit is conducting we have a simple loop where I = V/R still applies (ignoring the leftmost transistor collector current). So decreasing the coil resistance increases drain.

Your transistors are set up in a darlington configuration so you should be able to use a single darlington transistor. You best bet is to look up one which meets the specs of the circuit.

3. ### StayatHomeElectronics AAC Fanatic!

Sep 25, 2008
1,021
71
1. The current through the relay coil is going to be determined by ohms law in steady state, i = V / R. Therefore, increasing the coil resistance will decrease the current draw through the coil which in turn reduces the current draw from the battery, the battery will last longer. Decreasing the coil resistance does the opposite and will drain the battery faster.

2. I am not sure what the resistance of you finger is going to be. Also, what is the resistance of the coil that you are using?

3. You could definitely use an NPN darlington transistor. You could try something like an MPSA14. You will not have access to the point where R3 connects so that resistor will be eliminated. The Vbe(on) is maximum of 2 volts so R2 will need to be selected to make sure at least that happens when you touch the AB contact for the transistor to turn on the relay. You really just want to saturate the transistor to get as much power to the relay coil as needed. The Vce(sat) is 1.5 volts, so make sure the relay coil will activate with V(battery) - 1.5V.

4. ### mik3 Senior Member

Feb 4, 2008
4,846
69
R1 is to high for this application. You don't really need R1 but use a 1K for R1 just to protect the transistor in case the base touches Vcc.

5. ### burtS Thread Starter Member

Mar 6, 2008
29
0
Steiner96, Stay at Home, Mik3:
Many thanks for the comments, have ordered the darlington and will give it a try.
Mik3: I need a high value for R1 to minimize current across the subject (mice): I'd like to get that down to 0.2 ua or so and any suggestions for an improved (but still reasonably simple) circuit would be much appreciated. Present circuit becomes less reliable when R1 exceeds 10M or battery voltage drops below about 7v.
Burt

Feb 4, 2008
4,846
69