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I'm working on a device that illuminates a signal LED when a button on a remote is pressed. I have a working prototype based on a pre-fabricated board I bought on Amazon (see here), and it's nice! But now I'm toying with the idea of producing these on a larger scale, and looking for a less expensive solution.

The thing I keep running into is that every system like this one is relay-based, and for the life of me I can't figure out why. Couldn't the same thing be achieved with a transistor? Is there a generally less-expensive solution out there I should be looking for?

Spoiler alert: I'm very new to electronics design, please explain like I'm five.

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    \$\begingroup\$ Relays are a cheap and easy way to handle high currents and high voltages. Using transistors for this would be possible, but generally more difficult and more expensive. \$\endgroup\$ – Hearth Nov 22 '18 at 18:08
  • \$\begingroup\$ Is it more difficult/expensive in general, or specifically for high-voltage cases? My usage is pretty decidedly low-voltage. \$\endgroup\$ – Max Batchelder Nov 22 '18 at 18:10
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    \$\begingroup\$ "My usage..." Precisely. The fellow who designed the remote control board didn't tailor-make it for your use case. He made it for the most broadly applicable usage he could think of. \$\endgroup\$ – Nick Alexeev Nov 22 '18 at 18:11
  • \$\begingroup\$ This question boils down to relay versus transistor and the answer is clearly down to the intended application so read the data sheet of the relay based device and try and work out why they are targetting that market. It's not really a valid question as it stands because it draws in RF and radio when they needn't be. \$\endgroup\$ – Andy aka Nov 22 '18 at 18:13
  • \$\begingroup\$ Is there a different technology, like transistors, that I could be searching for instead of relays? From what I've found it's really only relays. I get that they're the cheapest/easiest/most generally applicable, but are they the only game in town? \$\endgroup\$ – Max Batchelder Nov 22 '18 at 18:14
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  • Relays have the advantage that they offer complete electrical isolation between the controller and the load.
  • The relay can be used to switch low-voltage signals or mains at modestly high currents.
  • Relays make a nice clicky sound.
  • Relays are reasonably cheap.

enter image description here

Figure 1. Solid-state equivalents of the relay boards are available by the million too.

The solid-state boards use opto-isolation between the logic side and the load to provide a similar level of insulation to the relays. Their advantage is that there are no moving parts to wear out. Their disadvantage is that different types are required for DC and AC switching operations.

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  • \$\begingroup\$ "nice clicky sound" is actually one of my complaints! In a perfect world I'd have something silent, I guess an opto-isolator would work well. I don't need the isolation, however, the rf receiver and the circuit it's activating are all operating in the same voltage range. \$\endgroup\$ – Max Batchelder Nov 22 '18 at 18:16
  • \$\begingroup\$ @MaxBatchelder Some relays are remarkably quiet! Had a case a few days ago where I initially thought I'd gotten a dud relay because I couldn't hear it click when I powered the coil--turns out it was just really hard to hear. \$\endgroup\$ – Hearth Nov 22 '18 at 18:19
  • \$\begingroup\$ @Max Look-up solid state relays, which don't make sound. More importantly, what are you planning to switching? \$\endgroup\$ – Nick Alexeev Nov 22 '18 at 18:22
  • \$\begingroup\$ All I'm switching at the moment is a single 3mm LED, from a 5v source (also powering the RF receiver) \$\endgroup\$ – Max Batchelder Nov 22 '18 at 18:27
  • \$\begingroup\$ @Max: The LED can be powered from the micro-controller output via a series resistor. See Ohm's law and resistor calculation. \$\endgroup\$ – Transistor Nov 22 '18 at 18:59
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A transistor requires that your circuit (the remote control receiver) is electrically part of the thing you are controlling.

When you work that way, you have to consider things like reverse voltage, collector emitter voltage, voltage and current spikes from the controlled circuit, etc.

Transistors aren't as robust as relays - it is relatively difficult to kill a relay, much easier to kill a transistor.

A relay electrically isolates the controlled circuit from the controller, which is a good thing. All kinds of bad things can happen on the output without damaging the controller.

An alternative would to use a solid state relay (SSR.) They combine the isolation a robustness of a mechanical with the non-mechanical switching of a transistor.

I haven't looked, but I expect SSRs cost more than a mechanical relay for the same voltage and current ratings.

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  • \$\begingroup\$ I can confirm that SSRs in general do cost more than electromechanical relays. Had to use them more than once. They aren't usually as robust as electromechanical ones either, certainly not against short-duration overcurrents. \$\endgroup\$ – Hearth Nov 22 '18 at 18:17

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