# 5V impulse relay

I'm looking for relay similar to Finder 27.01.8.230 http://www.farnell.com/datasheets/1785142.pdf but with 5V coil voltage.

I'm confused about naming and other parameters. What is the correct name for this relay? Latching/impulse/signal? What does single coil and double coil mean? There are lots of types and parameters and I don't how to choose the right one.

• At 5V signal voltage, wouldn't it be easier to use a binary counter, or a ripple counter, connected to MOSFETs, optoisolators, or triacs (depending on the nature of attached load) to accomplish the same thing, but with a longer component lifespan (and likely lower price tag)? Feb 8, 2016 at 5:01
• Could you be more specific? My goal is to use momentary switch to turn 230V device on and off. I would like to control it with 5V voltage. I'm beginner in electronics so it is hard to imagine how to use these components. Feb 8, 2016 at 13:05
• Ok, are you wanting to just control 1 device? Or 2 devices? Feb 8, 2016 at 15:34
• Let's say 1 device Feb 8, 2016 at 19:55

I think this is a better/simpler solution. The toggling behaviour you get from a flip-flop, in this case a "J-K" 74HC73 which with both inputs wired high, will toggle every time you clock it.

simulate this circuit – Schematic created using CircuitLab

R1/C1 is debounce circuit for SW1, which will cause the flipflop clock to go low for about 50ms each time you press the button. M1 provides drive current (up to 100mA) for the coil of the relay and D1 protects M1 from overvoltage when turning off.

RLY1 has a 5V coil (they're usually about 80mA) and contacts rated for your load. If you need a bigger relay with a coil that draws over 100mA, then use a bigger FET too like an AO3400 or similar.

R4/C3 is the reset circuit, which resets the flipflop to low (output off) on power-up. It will be in reset for about 10ms. D2 will quickly discharge C3 when the power is removed, forcing a clean reset.

C2 is a decoupling capacitor; you put it directly across the power terminals of the flipflop.

Edit: for completeness, I'll assume you may also need help with the 5V power supply. Look for a 7805 and read the datasheet/appnote for that, it will give you a clean 5V to run this circuit from any source between about 7V and 20V. The 7805 will need a heatsink if the source voltage is high and/or the relay coil draws a lot of current, but that's a whole other question and readily googleable.

• I'll give that a +1. While I'm certain that my circuit would accomplish OPs stated requirememts, yours can do so at least as well, and with a lower component count. I can't argue against that. :) Feb 12, 2016 at 5:00
• Thanks for your solution, I tested it and it works great, but as you said I have probably problem with power supply. Currently I'm using 12V 1A adapter with LM7805ACT voltage regulator. Everything is fine except relay which is not switching. I tried SDT-S-105DMR,000 and also srd-05vdc-sl-c. Do you think that problem is with FET and relay doesn't have enough current? I will also try AO3400 as you suggest, but first I have to get one. Feb 19, 2016 at 7:25
• how is it fine if the relay is not switching? That relay does look like it has a pretty heavy coil (50 ohms, 100mA) so a bigger FET or BJT would help. Theoretically the 2N7000 should be capable of driving a 100mA load though. D1 being backwards (note it should be reverse-biased normally) will stop it working and cook the FET. Feb 22, 2016 at 0:12
• Do you have input and output capacitors on the 7805? Feb 22, 2016 at 0:13
• Yes,capacitors aren't the problem. I will try bigger FET or relay with smaller coil Feb 22, 2016 at 21:54

Here's a circuit that should accomplish what you're wanting. Every time S1 (whatever is generating your pulses goes in place of S1) sends a pulse into the circuit, the relay will switch off-to-on, or on-to-off.

EDIT:
The DPDT relay, triple-latch buffer, and the attached network of resistors, capacitors & transistors (now added) will reset the entire circuit to 'off' state for your 230VAC circuit any time there is a power fault. You may want to attach a large capacitor to your 5VDC supply in order to keep the circuit from resetting during transient power losses, if you notice nuisance tripping of the reset. /EDIT

All part numbers are given for parts that are available on http://www.digikey.com I selected them to be compatible with each other in this circuit and inexpensive (I think only 2 pieces were over \$1.00USD/ea).

• Thank you, I will try it as soon as I receive all parts. Feb 9, 2016 at 9:08
• I see you want the circuit to always power up in 'off' setting. Does your 5V power supply have a battery to keep it from being reset when the power goes off? If not, we might need to modify this circuit at bit, as it may be in 'random' state upon powering up. Feb 9, 2016 at 13:54
• I will try to clarify it. After power up this circuit, AC device is off. When S1 sends the pulse, AC device is turned on. When S1 sends another polse AC device is off. So whenever we reset the circuit (remove power supply), AC device should be off and also when we power up circuit again, AC device is still off. Additionally my 5V power supply don't have a battery. Feb 9, 2016 at 14:07
• @Matus Ok, that makes the circuit a bit more complex, and would not necessarily have worked with your first relay (it most likely would have powered up in last state). I'll do some research/calculations, then update the schematic. Feb 9, 2016 at 14:22
• How soon after power-on will the device have to be ready for input? I.E. Will a ~5 second delay before the fkrst 'turn on' pulse can be accepted cause you problems? Feb 9, 2016 at 14:28

That device is a step relay, pulses of the same type step it though the available states, an alternating relay is a similar device (but with dual-throw contacts)

A brief search didn't find any suited to DC drive at 5V

Latching relays use pulses of different types to select the different states. single coil latching relays use opposite polarity pulses, double coil latching relays use distinct coils for on and off (but will often respond suitably to pulse polarity too).

for 5V operation you could build a step relay from a latching relay and some ordinary relays, but I would look for an alternative solution

• Thank you for your clarification, do you have an idea how to build it from these items or what could be an alternative solution? I can't find anything useful. Feb 8, 2016 at 12:57
• alternate solution depends on the constraints of your unstated requirements... eg: does it need to remember the state during a power cut? Feb 9, 2016 at 9:21
• no, after power off and on again, device should be turned off. Feb 9, 2016 at 10:29