I just bought this 8 channel relay module on Aliexpress, and I'd like to try to trigger it using an air coil and a moving magnet, but I'm unsure how to connect the coil to the trigger and collect the required voltage to trigger it as the magnet leaves the coil.

enter image description here

You can trigger the SSR's individually by simply touching the DC- to any of the 8 single trigger terminals directly. I could put a reed switch in between these two connections, hold it in place as the magnet moves by, but I don't want to do that.

enter image description here

I really want to know if this is possible with a coil. The size of the coil and magnet are not important right now, I'm really just confused about whether this is even possible or not. I'm not an electronics guy and I've watched way too many YouTube videos trying to figure this out on my own. Here's the amazing circuit design that I've come up with so far.

enter image description here

I know it's missing specific values, but that's the next thing I have to figure out. For now, I'd be really happy with a push in the right direction.


UPDATED: Dec 29, 2021

I am still struggling to understand how I can do this without a reed switch. It's easy on a full-size SSR because you have a + and - terminal for the trigger, but this relay module only uses one terminal for its trigger. I also don't want to use an Arduino or additional power source to set off the trigger. I just want to trigger the relay as a magnet moves away from a simple air coil, which should generate a - voltage. I've been trying to figure this out on everycircuit.com. Here's a link: Using AC to Pulse a Relay Module

  • \$\begingroup\$ Those relay boards use normal mechanical relays, not Solid State Relays. They do have optocouplers to drive the relay, so your drive circuit doesn't have to deal with the relay coil current. \$\endgroup\$ Dec 26, 2021 at 2:37
  • \$\begingroup\$ These are relay/solenoids. It is easier to trigger with coil current than external coil magnetic H field. So why? \$\endgroup\$ Dec 26, 2021 at 3:12
  • \$\begingroup\$ Thank you @Peter Bennett, I know. If you look closely at my last image, I don't connect the source voltage in any way to the trigger. Did you have a suggestion on how I can do this with a coil though? \$\endgroup\$
    – Vince
    Dec 26, 2021 at 4:44
  • 1
    \$\begingroup\$ Magnet moving over coil will produce current proportional to rotational speed, so it will only work if it is turning fast enough. If you want it to work when it is not turning or turning slowly, use a magnetic reed switch. If you want it to work only when it is turning fast, the coil is a good option. \$\endgroup\$
    – bobflux
    Dec 26, 2021 at 9:39

3 Answers 3


A magnet pole moving over a coil causes a bipolar voltage pulse that depends on the speed of the magnet; there will be a peak, then a valley, each having the same volt-seconds product (area between the V versus time curve and the V=0 axis).

The nominal best practice in such situations is to use the complicated pulse to trigger a one-shot, such as a '555 timer, and make a short known-voltage pulse that drives or sinks current to operate the relay board.

So, an easy way to do that(I'm assuming active-low and that there's a pullup in the relay board) is


simulate this circuit – Schematic created using CircuitLab

A simpler system would be


simulate this circuit

but that can only briefly activate the relay; you need a good fraction of a second before the relay completes a closure.

  • \$\begingroup\$ I don't have an electronics background so I don't understand what the clear arrows are for @Whit3rd. I presume they are just the two ends of the same coil but it doesn't seem to work for me. Is the 'output' supposed to run into the trigger on the board? \$\endgroup\$
    – Vince
    Dec 30, 2021 at 1:16
  • \$\begingroup\$ @Vince: the clear arrows indicate the "Ground/zero volt" point in the circuit - they rarely indicate an actual connection to the earth, but just mark the point we decide to call "Zero Volts", and use as a reference when measuring voltage elsewhere. All such points are connected together - you could just draw a wire between them, but it is sometimes clearer (to those who know) to use the Ground symbol rather than drawing all the ground connections. \$\endgroup\$ Dec 30, 2021 at 1:29
  • \$\begingroup\$ Thank you @PeterBennett, that's exactly what I thought. But after testing this circuit on everycircuit.com it doesn't seem to do what I need either. Check the third circuit down on the left side. What am I doing wrong here? \$\endgroup\$
    – Vince
    Dec 30, 2021 at 2:36

I'm still not sure if this is even possible with this board, but I've realized something along the way.

I thought the '8 Channel Relay Module' was the same as an '8 Channel Solid State Relay (SSR)'. I understand now that they are not. The '8 Channel Relay Module' can only be used with an input signal from something like an Arduino or a 'Reflective Photoelectric 3pin IR Infrared Sensor Module'. The '8 Channel Relay Module' also operates much slower than I want, so I'll go back to working with good old fashion SSRs with 4 terminals.

Thank you all for trying to help, I told you I don't know what I don't even know yet. lol


There's a lot to process here. I'll do my best.

1: as far as the diagram goes, I don't think you're connecting your transistors how they're supposed to be, at all. I'm guessing you're trying to amplify the coil's signal.

2: I'm guessing from the pictures that you have a rotating table where the magnet is placed and it travels above the coil. I don't think that having the magnet so far away would generate enough voltage. You would require enough movement at the right speed and polarity to actually generate a trigger signal.

I would suggest using the reed switch. Take, for example, the size of the filaments of the reed switch, It's designed like that for the form factor, and also because you need huge (relatively) coils and/or magnets to create working voltages.

  • \$\begingroup\$ I'm not trying to amplify it. As a magnet passes over a coil it creates a waveform. I'm trying to send a trigger pulse to the base of the NPN transistor when the magnet starts to move over the middle of the coil where it generates a - voltage. As for the first photo, that's just a test area I built, and the coil is normally directly under the magnets as they spin. That coil generates 39 Vpp when the magnets are spun at 200 rpm. I just don't know how to connect it to set off the trigger. I've used a reed switch in the past, I don't want that this time. \$\endgroup\$
    – Vince
    Dec 26, 2021 at 4:52
  • \$\begingroup\$ @Vince: What are you going to do with the output of the relay? I don'l think a relay will work too well being triggered at more than three times a second (200 RPM). \$\endgroup\$ Dec 30, 2021 at 1:35
  • \$\begingroup\$ @PeterBennett, should I go back to using a normal SSR? I thought these relay modules were just like SSR. \$\endgroup\$
    – Vince
    Dec 30, 2021 at 9:08
  • \$\begingroup\$ Tip: Markdown syntax can handle 1. or 1) and convert it to properly indented lists. 1: it doesn't recognise. There is a help button as well as list formatting buttons on the editor toolbar. \$\endgroup\$
    – Transistor
    Dec 30, 2021 at 13:47
  • \$\begingroup\$ @Vince: The relays on those modules are normal mechanical relays, but the modules include driver circuits so they can be controlled from a microcontroller or logic gate output. Whether they are appropriate for your use depends on what you are doing with the relay outputs, how often you expect the relay to operate, and how long you want the contacts to remain in the "On" position. \$\endgroup\$ Dec 30, 2021 at 16:52

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