Linking transistor switches to control an event

Thread Starter


Joined Feb 10, 2012
I can't argue myself which of the following two arrangements is better.
For example pumping is controlled by lower and upper tank of water levels.
Would the current in the relay coil remain same for A and B?


Joined Sep 17, 2013
The relay coil current could be almost the same for A and B, but you would need to add resistance in series with the inputs to prevent the magic smoke escaping.


Joined Sep 24, 2015
Diagram A is a "2 input Gate". Meaning the relay will only close when both inputs are high (have or see power). Circuit B is more of a buffer. Input 2 drives input 1 which drives the relay. Aside from the need to add resistors to your circuit, it would be advisable if you would give us the details of what it is you want to do. It's sounding to me like you want one input to turn the pump on and the other input to turn the pump off. So that if you're monitoring the water level in a tank, when the level gets so low the pump is turned on. When the water level reaches the high limit the pump turns off.

There are a number of different transistor types you can use. Using the RIGHT ones is very important. Also, you need to understand that your "Inputs" also need a ground (or common) reference point. Meaning there should be a low voltage present so that when the low limit sensor switch no longer sees a voltage (because the water level is too low) it needs to activate a latching relay so that the tank can be pumped back to the full point, where a second sensor detects the voltage and then shuts off the pump. There's more to it than just detecting when the water is low and when it is full.

Give us more info and we can steer you in the right direction. (did I use the proper form of "steer"?)