I need to detect if a monomode fiber loop is intact. To that end, I have a laser diode emitting max. 10 mW (adjustable) at 1310 nm.

At the receiving end, there is a matched photodiode (Laserscom.com PDI-80-2G-K) which gives about 1 mA per mW when I measure directly over the output leads.

The laser diode and the photodiode are inserted in the fiber loop and work as predicted.

The problem to solve is how to use the diode so as to have a relay open when the laser light is lost.

Looking at a commercial simple dusk switch using an op-amp and a BC547 it appears that it might be possible to use the diode instead of the LDR in front of the op-amp provided that the circuit is adapted accordingly regarding resistor values etc.

The diode data are: max. reverse voltage 20 V, operating voltage 3-5 V, responsivity 0,96 mA/mW at 1550 nm (slightly less at 1310 nm).

How could the circuit below be modified to use the photodiode (the LDR has been replaced with the photodiode in the drawing)?


  • \$\begingroup\$ I already have the diode which is fitted with monomode fiber pigtails and the necessary SC connectors. It was not easy to find a laser transmitter to connect to the monomode fiber loop. I found one in Byelorussia and it could be delivered with a matched diode. So I restrict the question to circuits with a photodiode input and a relay output \$\endgroup\$
    – jonasolof
    Jan 19, 2020 at 17:49
  • 1
    \$\begingroup\$ Do you care about latency , dropout time , hysteresis, drifting levels thermal effects, avoiding supply ripple? Variation in Tx power levels? Then add to YOUR missing Design Spec. \$\endgroup\$ Jan 19, 2020 at 19:28
  • \$\begingroup\$ unless you define anything else such as above, Murphy's Law may happen to you. Like an oscillating Relay where the level cuts in and out that quickly burns out due EMI pickup from the relay kickback. Generally rise time in a PD due to diode cct capacitance is dV/dt=I/C thus turn off fall time is slower yet both depend on thresholds and RC time constant with noise margin and tolerances. It is really simple once you define these levels and times with impedance of signals and noise. List what you know and TBD for the rest. or dont and just play with it till it seems to work for now. (hmm...) \$\endgroup\$ Jan 19, 2020 at 20:42
  • \$\begingroup\$ Tony, thanks for your warnings. Unfortunately, I lack the knowledge to take what you listed in account. I'll make a protracted test before integrating the circuit in the alarm system. \$\endgroup\$
    – jonasolof
    Jan 20, 2020 at 12:08
  • \$\begingroup\$ Test by Trial and Error without specs? \$\endgroup\$ Jan 20, 2020 at 12:11

3 Answers 3


If you can find a a phototransistor sensitive to 1310nm, you should use that intead of a photodiode here. Photodiodes are required for high sensitivity, high speed, high linearity circuits but also much more TIA circuitry. There's not much point here for your intended purpose. If you use a phototransistor you might be able to use it to drive the relay coil directly but I somewhat doubt it. Instead you can use the phototransistor (in a darlington-like arrangement) to drive another transistor to drive the relay coil.

If you want to fine tune the switching threshold you would need a comparator. By the way, the opamp you posted is being forced into use as a comparator which is not optimal. It should preferably be a comparator with hysteresis or else you might find yourself with unstable behaviour.

Just place the resistor above and the photodiode below, or vice versa across the power rails to match your comparator logic state and use the center node as your input to the comparator. You won't need all that extra stuff in front of the opamp if that's the case.

EDIT: Since you are using a laser with your photodiode, you may be able to use its output directly, as you would a phototransistor, if the laser diode can excite the photodiode enough.

Otherwise you will need to build a transimpedance amplifier (TIA) around it.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Still, I see many designs on the web where a photodiode (not a phototransistor or LDR) is used with an opamp and a following transistor to drive the relay. Only I lack the knowledge to design a suitable circuit myself. I know that photodiodes are over qualified for the purpose. I don't need linear response, just on/off. Als, I haven't seen any photo transistors with monomode fiber pigtails and SC connectors. There wouldn't be any market for them. \$\endgroup\$
    – jonasolof
    Jan 19, 2020 at 17:59
  • \$\begingroup\$ @jonasolof Are those designs you saw where a laser directly illuminates a photodiode because if they were not they probably would need a TIA. \$\endgroup\$
    – DKNguyen
    Jan 19, 2020 at 18:01
  • \$\begingroup\$ OK, I'll try putting the diode as you suggested. What about the minus side input of the opamp? Should it just float? \$\endgroup\$
    – jonasolof
    Jan 19, 2020 at 18:06
  • \$\begingroup\$ @jonasolof No, use your part of the circuit for that. I was just lazy. To test just wire up the resistor and photodiode with power and illuminate it. See how much voltage output you get. If it turns out you get enough voltage but not current, you can use an NMOS instead of a BJT (for the lower circuit. for top circuit opamp inputs should be quite high impedance already so current shouldn't matter). \$\endgroup\$
    – DKNguyen
    Jan 19, 2020 at 18:20
  • \$\begingroup\$ The band gap of silicon devices means they are not sensitive to 1310nm light, they are only good up to ~1000nm. The only devices normally available for 1310nm will be indium gallium arsenide photodiodes. \$\endgroup\$ Jan 19, 2020 at 18:53

The problem to solve is how to use the diode so as to have a relay open when the laser light is lost.

Either use a normally-closed relay (a form B relay or the NC contact of a form C relay), or switch the inputs to the comparator.


The photodiode, a InGaAs PIN, is driven by a laser diode at 1310 nm through a monomode fiber loop with about 5 mW power. This gives 0,48 volt out from the diode over a 1 mohm resistor. This is enough to direct feed the LM324 op amp in a modified 7 euro Velleman MK125 dusk relay kit. As it is for an alarm, the relay should be NC. Therefore, the inputs to the opamp have been shifted. When there is light through the fiber, power draw is only 0,7 mA from the 12 volt supply. Power draw when the relay is activated depends on the coil resistance.

The photodiode is not used in the usual reverse voltage mode. This is a crude solution but it works well.

The disadvantage is that the laser diode plus optodiode fitted with fiber pigtails and SC fiber connectors are expensive, about 240 euros plus shipping and VAT. An alternative is to use TP LINK fiber to Ethernet converters such as MC111CS and MC112CS. When the fiber link is intact, a led in each unit provides 1.9 volts over the led. This can be connected to an external circuit, in my case a transistor 2N2222A preamp followed by an optocoupler (TIL111) and a transistor 2N2222A to control the relay.


  • \$\begingroup\$ If that works, it's more by luck than judgement. It relies on the photodiode modulating bias current of the inverting input of the LM324. For that particular opamp, this might actually work, due to the inputs of the LM324 being PNP transistors, but it would fail with any FET input opamp. \$\endgroup\$ Oct 26, 2022 at 1:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.