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My project uses light dependent resistors (aka photoresistors or CdS cells). All the ones I can find for sale from the usual suppliers are through hole type. ie. they have two wire legs.

The PCB I'm working on currently consists entirely of surface mount components. I'd like to place two LDRs at the edge of the board at a right angle. I could just solder them manually but I enjoy the exercise of designing for manufacturing even while I'm still prototyping and adding through-hole components would introduce another step.

Best idea I could come up with so far is to use a right angle surface mount header and then whoever assembles the final product just pops two of them in, probably after trimming the leads. But I tried with some standard 0.1" spaced female headers and it results in a rather loose fit.

I was thinking there might be some screw terminals or something press-fit that would result in a more secure connection but so far my search skills are failing me.

Any ideas?

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    \$\begingroup\$ The "Cd" in CdS stands for cadmium, which is pretty toxic. So a CdS photocell isn't going to be RoHS compliant. So if you really want to be manufacturable, design it out. \$\endgroup\$
    – TimWescott
    Feb 6, 2021 at 16:41

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The fact that there's hardly any commercially available surface mount LDR should tell you something about the obsolescence of these parts, in my humble opinion.

Your best idea introduces an extra, nonstandard step, plus another mechanical component, so that's definitely way worse than introducing a through-hole component.

Considering this is an exercise in "design for manufacture", then the design problem becomes "what to replace a component with a bad availability and unsuitable mounting with?", and the answer is, typically, "a photodiode and an amplifier replaces an LDR with an amplifier".

Then your problem becomes relatively easy:

Side-Looking photodiode

There's side-looking photodiodes; you need to operate that thing in reverse bias with a series resistor, and amplify the voltage across that, which typically just takes an opamp and two resistors. Even better would, if you need the bandwidth, be a trans-impedance amplifier (TIA) configuration for the opamp.

Since you're probably doing something with your LDR resistance, too, that might not even be extra effort.

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  • \$\begingroup\$ Thanks! The LDRs would have gone straight into op-amps anyway, so should be easy enough to adapt the circuit. You're right - the fact that there aren't surface mount alternatives should have been a clear sign. \$\endgroup\$ Feb 6, 2021 at 15:30
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Well, LDR are not often found around these days, photodiodes/transistor have better performance in 99% of the cases. In fact IIRC there is a ban on CdS cells (because of toxicity) except for audio optocouplers.

You could try using a screw or spring terminal block, but the terminals are usually quite fragile

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