Sunday, 17 April 2016

Flip-flop or bistable latching using relays.

Hey guys,

Since I have plenty of relays laying around and dont know what to do with them, I decided to test if using this relays can mimic the flip flop actions of solid state devices such as transistors.

I know that there is already a latching type of relay made into a single package 'latching' relay in the market, but since I have different type of relays which functions as mere simple switching conveyor, I have to use my brain on how to configure these relays into a circuit that can mimic flip flop action.

I came up with two different types of circuitries; design A incorporates two different contacts for SET and RESET actions while design B incorporate only single contact to implement SET and RESET actions.

For readers who don't know what is a flip flop circuit, it is a type of circuit which retains the previous state of its output. For example , say if you have a flip flop circuit operating an LED. The ON and OFF conditions of the LED corresponds to the output state of the flip flop. If we switch ON the circuit, the LED lights up, of course. If we release it back, the LED remains ON. If we apply another press on the switch, the LED goes off. Again if we release the pressing, the LED remains off. This is what flip flop actually does. And due to this ability, it has an ability to store previous input without a need to retain the flow of current, which reduce power consumption and increase operational efficiency. This circuit is used extensively in computer memory and other types of machines that utilizes memory functions.

Here is the circuitry for flip flop using relays;

Update (19th Apr 2016): Unfortunately, Design B proposed above does not work (perhaps it will works at higher input power however higher input power will damage the LED. Updates on 21st Apr 2016: higher power also does not work!). Therefore a revision was made to Design B (see below), but at the expense of power loss since at this new design power is conducted at the triggering flip flop circuitry (Relay 1 and Relay 4) during ON state of the LED. Hopefully this new design will work (update will follow soon...)

Update (21st Apr 2016): Unfortunately, the amended Design B also does not work. I experimented for hours later on and found that the following design for single toggle works, but with oscillating output (LED is blinking). In order to slow down the oscillation, I added up a big capacitance capacitor across Relay 3 coil and the toggling eventually works, The higher the capacitance the slower the oscillation and the easier for you to toggle. Here is the schematic;

Update (22nd Apr 2016): I found a rather simpler circuit for toggling using relays here.
I have made some changes on the proposed circuit such as removing the resistor, introducing LED output and also using two SPDT relays instead of DPDT relays. I have tested and it works excellently, without oscillation. Here is my variant of the circuit.


  1. Hi, I have tried your design "C" circuit and while it works, I have found there is a high current draw at relay 1 when unlatching as the COM and NC contacts are a dead short to ground. OK for a push button switch but I need to do this with another relay and will fuse the contacts together in a short time. I'm trying to make a divide by 4 circuit by using two of these flip flops which are basically divide by 2 circuits. Any thoughts on the high current draw? This is the only circuit I have found that doesn't oscillate.

    Regards, Danny.

    1. Hi. I think perhaps your relays has very low resistance compared to the wire resistance. What I can suggest is to use relay with higher coil resistance. Or if does not work, just refer to the link where I originally obtain the circuit