Martin's corner on the web

Trying out bistable/latching relay

I purchased a Ciseco bistable/latching relay few months ago for the sake of trying it out. I don’t have a particular project that requires one, just wanted to toy with it. If you haven’t heard of this type of relays, they are pretty cool because they only need a brief pulse to set and another one to reset. A ‘normal’ repay requires keeping the coil energized to have it switched for comparison. So briefly pulsing the set or reset pins turns the relay on or off without further power requirement. This makes it ideal for battery operated nodes; The relay works on 3-5V systems, so ideal..but once I built mine, I could not drive it from battery operated node. It did work while the node was powered from the USB cable, but not when I had it running on battery. I hooked the power line to the oscilloscope and noticed a massive drop in voltage when the coil was getting energized – it dipped from 3.3V to 2.4V, probably causing the transistor issue. I asked for support and @Miles quickly replied that I should try swapping the through hole 2N3904’s with SMT BC817 transistors. The PCB has almost each component dubbed with SMT footprint, so it could be build either with through hole or SMT. I didn’t have the transistors so I had the relay collect dust in a drawer for a while, until recently I decided to do the patching. I replaced the transistors and gave it another try. Now all worked out nicely, the relay was making a happy ‘click’ each time I set or reset it. It would work with pulses as short as 20ms.

I am glad to have it working, may come in handy in some future project.

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