I have a prototype circuit set up that uses eight 2N2222 transistors set up as four Darlington pairs to amplify the signals from four photodiodes. Circuit is working well on a breadboard but I’d like to reduce the number of components that I’m working with before building a custom board. Would the ULQ2001A Darlington Array (ULQ2001A STMicroelectronics | Discrete Semiconductor Products | DigiKey) be a reasonable substitute for the eight 2N2222 transistors? I’ve tried the circuit with a ULN2003AN Darlington Array (which I happened to have on hand) and it didn’t work – I presume because of the 2.7kOhm resistor in line with the base on the first transistor of the Darlington pair…
The ULQ2001A looks to be a reasonable substitute, but I’m not familiar enough with them to understand what the two resistors between the input/first transistor’s emitter/ground are doing and how they would affect the circuit.
Diagram of my (working) circuit, for reference:
Thanks!
The resistors in the Darlington are there primarily to discharge built up capacitance. It’s called the “Miller Effect” if you want to Google that for a complete explanation. ULN2003AN has a 2.7k resistor on the base, which most likely is the cause for not working in your circuit. ULQ2001A does not have a base resistor, so it very well may be a direct drop in. Unfortunately, we don’t have any Darlington arrays that don’t have resistors between the transistors.
Thanks Andy! I did look up the Miller Effect – thanks for that info!
I realized that the two internal resistors were similar to the 1M resistor I have in my circuit – which I have to “bleed off” low levels of current that are present when the photodiode picks up stray light that is present when it’s main target is off. I changed my 1M to smaller resistors set up in the same way as the internal resistors (i.e. base to emitter/base to ground) and it draws off too much current such that the photodiode never turns on the transistors even when it is “on”. So it looks like the ULQ2001A probably won’t work for me. I’ll just have to build it the old fashioned way! 
Hello Lawrence,
Your 2N2222 Darlington connection is a good solution. It’s simple and it works. However, it may not be the lowest cost solution.
Assuming you wish to stay with through-hole components, you may want to consider this CMOS hex inverter:
You may also want to consider using a microcontroller such as a the popular Arduino. It’s excessive for your described application but may offer additional flexibility such as an alarm if one photodetector is absent. I mention this particular family of microcontroller as they have a large user community to help get you started.
Thank you for contacting this DigiKey community. Kindly share your results. Pictures are especially welcome.
Best Wishes,
APDahlen
P.S. Since your circuit needs are not particularly tasking for the 2N2222, you may want to switch to a less expensive transistor such as the 2N3904.
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