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Electronics => Projects, Designs, and Technical Stuff => Topic started by: girishji on September 23, 2023, 06:38:45 am

Title: Removing voltage spikes in a Phototransistor circuit
Post by: girishji on September 23, 2023, 06:38:45 am

I have a IR LED and PT (phototransistor) setup where LED is switched at moderate frequency. PT is connected to an amplifier circuit to increase response time. There are 5 PT's connected in parallel. There are also 5 IR Led's, one per each PT (not shown).

Only one of the 5 LEDs is powered at any given time for 10-15us, followed by the next one and so on. So only one of the 5 PTs is active at a given time. But one of the LED is blocked occasionally to get voltage transition measured across Vout (Vcc is 3.3v). I am using a RP2040 to turn on LEDs through software.

Whenever IR LED is blocked, oscilloscope registers sharp spike of  50v or more at the edge of the square signal. In fact, it registers sharp spikes whenver there is a sharp edge.

Any idea what is going on here? Any solution how to avoid damage?

Title: Re: Removing voltage spikes in a Phototransistor circuit
Post by: tggzzz on September 23, 2023, 07:05:26 am

It is unlikely voltage spikes are related to the phototransistor. It is more likely that it is related to connecting the scope to your circuit, possibly due to where you are connecting the probe shield, possibly due to the PSU in your circuit.

I suggest you use a voltmeter to measure voltages between the probe's shield and various parts of your circuit.

(Oh, it isn't important yet, but "numbers not adjectives" hence "moderate frequency" => "x Hz" :) The 15µs helps, of course)