The Photomod Optical Coupler


The photomod opto isolator was the isolation device of choice (only choice) a generation ago. Since it predated the LED, the illumination was accomplished via incandescent or neon lamps. Since the load circuit voltage was often line voltage (115 or 230VAC), the cadmium-sulfide photocell or LDR (Light Dependent Resistor) was the sensor of choice –while photo-transistors were available at the time, I never saw one used in a photomod.

Schematic of the Photomod Optical Coupler

Photomod Schematic

Applications were numerous: These were used to drive relay coils etc. One interesting application was an audio modulation limiter /attenuator that was used in radio transmitter audio line and level equipment –when the modulating signal became excessive, the lamp turned on, the photocell dropped in resistance and very quickly attenuated the signal –being resistive in nature, the photocell introduced no harmonic distortion (unlike the amplitude clipper).

However, with the advent of the LED, the photomod quickly fell into disuse. Today, the only photomod devices available are found on eBay and the surplus market –and for a price… The only vendors that made them (that I know of) were Sigma and Clairex –both of whom are now defunct. If you come across any in your travels, grab some for your bag of tricks. I could find no datasheet for the Sigma 301T1-PC device that I have on hand, so I did some experimentation and found that it has an incandescent lamp and that it turns on at 5V @ 15mA so that it can be driven via a 5V logic driver.

In this article, I wish to revive interest in this useful device so it can be used as an On /Off control for a TRIAC or SCR line voltage switch. Since they are no longer available, I will show how to make a DIY photomod using discrete LED /photocell devices.

Ebay Photomod Screenshot

eBay photomod screen shot

Photomod photo gallery

Clairex photomod short form specifications that also includes some LED photomods –note that this was hard to locate –also note the high voltages that the photocell can support (one is even rated for 3500VAC!). (clairex datasheet.jpg)

DIY Photomod

I am not the first one to attempt this –check out this link. In making your own, select a photocell that can support line voltage –I recommend the Advanced Photonics (API) PDV-P7002 device that is available from DigiKey for $1.41. I used a high brightness white LED and a piece of plastic toilet fill tubing. I had to bore out one end so that the TO-5 photocell that I had on hand was a press fit.

DIY Photomod gallery

Testing indicated surprising sensitivity. Of course, a series resistor is required to limit LED current. Also, the grey tubing that appeared opaque was actually not opaque –I had to wrap it with black electrical tape to block the ambient light.

Transfer function graph

Photomod Transfer Function

Testing on my SCR Dimmer Control

I could turn on the light bulb load by simply connecting a 3V battery to the LED via two 47Ω limiting resistors. Check out my Simple LED Tester. The photocell was connected across the potentiometer of my SCR Phase Control Dimmer. It provided an isolated means of turning the load voltage On and Off. Response tends to be slow because the photocell must increase to a high resistance to completely turn off the dimmer –this is just a fact of life when using these things. Note that no phase control is provided…yet –I am working on that one.

Photomod experiment

For the future

Isolated phase control input for my SCR Phase Control Dimmer


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