Trying to generate a simple 2-transistor circuit in which an animal subject on stainless steel floor acts as a switch when it contacts a stainless drink tube. Subject must experience no more than 1ua. Two circuits (attached) found in surfing web: both work but unclear what function is served by additional resistors in T1 circuit.
Questions: are there any disadvantages in using circuit T2?
should I diode the relay coil?
relay specified works well at 3 - 9 volts and operates many thousands of time with 9v battery but should I consider a different relay?
Know that op amp and other ICs can be used but prefer the more simple 2-transistor circuit.

 

The 10MΩ resistor between the base and -9v on the T1 circuit will make it less susceptible to false triggering due to static and stray electric fields in your lab environment. It also makes it a little less sensitive to touch. We use a similar home-brew circuit for some of our touch fountains, but operating on a 28vdc-powered Med Associates system.

Attached is a circuit diagram I made while doing some troubleshooting. The design was not mine, and the person that made them left no schematics. The zener diode and 3.3K resistor power the input stage at ~6v. The first two transistors work pretty much as yours, except the second one inverts the signal. The darlington is a level shifter for the 28v output to the system.

The rest of our fountains are also home-brew, but were designed around TL084/LF347 op amps. The transistor version is less complicated and they all seem to do the job.

Ken

 

Thanks for the reply: assume you prefer the T1 circuit. What purpose is served by the 1K resistor? After adding the 10M to the 'lick' line I found that the circuit worked better changing the 1K to 47K.
Burt

 

The 1KΩ resistor serves as the emitter load for the first transistor in T1. Adding the 10MΩ reduces the current to drive the first transistor (good for the rat!) , and increasing the 1KΩ to 47KΩ increased the voltage gain of that stage to compensate. The 1KΩ is not needed in the darlington configuration in circuit T2. Just two different ways of making a two-transistor amplifier circuit.

Ken

 

Ken:
Again, many thanks for the comments. Comparisons here suggest that the T1 circuit is more stable/reliable and while other values for resistors could be tested the ones used are doing the job and the circuit easily detects a mouse making contact with a water tube. Unless I'm overlooking something obvious to you, I'll go with the T1 circuit.
Cheers,
Burt

 

Go for it!

Is this a student project?

Ken

 

It's probably no big deal, but I don't see a protection diode across the relay. The diode prevents voltage spikes from the coil from damaging Q2. It's application is something like this (I'm using a convenient schematic to show configuration)...

 

Always a good idea Bill! For that low powered relay it's probably is not critical, but there should have been diodes across the solenoid valves in the ones in our labs...again not my design.

Ken

 

Go for it!

Is this a student project?

Ken

Ken:
Not a student project: the device will be used in on-going experiments with mice. Did not think the diode was needed but, as suggested, will add it. Many thanks for the replies.
Burt