Intermittant OP Amp Problem

[Chrisd]

I have an IR detector circuit configured for line-following robotic use. (Schematic attached) This circuit feeds an Atmel AVR microcontroller - Works fine - except in cooler weather it just ceases to work. Breath on it, hold a soldering iron near it or even just leave it connected to power for 20mins and it comes right - for awhile. Even a strong light or holding the Op amp between two fingers will wake it up after a fashion.

I have spent time working through the usual power rail and component value changes in an attempt to recreate the problem at will, but no luck - The fact that the board wakes up after handling etc makes troubleshooting difficult. I have several of these boards and they all exhibit the same problem, but some are worse than others.

I tried putting one in the freezer overnight, but it worked with no problems in the morning. This made me wonder about humidity?

Has anyone got any suggestions as to cause or how I might recreate the problem at will?

 

[Roff]

You have the threshold set at 3.9v, which is above the high input common mode range of (Vcc-1.5v). This may make it sensitive to the phase of the moon, if you are right on the edge.
Are both comparators of the LM393 used? If not, you need to tie one of the inputs on the unused section - either one - to GND. Not doing so may upset the internal bias of the used one.
BTW, LM393 is not an op amp, it's a comparator. You can sometimes use an op amp as a comparator. I have seen a comparator used as an op amp, but only in special cases.

 

[Chrisd]

The Second Comparitor has both inputs tied to ground.

The Threashold is set at 3.9 v as this is the appropriate point for the IR detector. I don't fully understand what you mean by "above the high input common Mode Range", but are there any changes that might remove this from the suspect list?

 

[Nigel Goodwin]

Are you sure it's not been upset by visible light?, IR phototransistors have reduced sensitivity to visible light, but they still respond to it.

 

[Chrisd]

I should have mentioned that in my preamble - Yes Visible light can alter its performance, but the temperature issue manifests itself in a shrouded environment, Daylight and windowless Lit rooms.

 

[Roff]

The Second Comparitor has both inputs tied to ground.

The Threashold is set at 3.9 v as this is the appropriate point for the IR detector. I don't fully understand what you mean by "above the high input common Mode Range", but are there any changes that might remove this from the suspect list?

LM393s are not guaranteed to work if the input voltage is above (Vcc-1.5v). Your input voltage is at (Vcc-1.06v). It may or may not work. It may work sometimes. To solve the problem, change your threshold voltage so that it is below 3.5v, and, if necessary, change the bias on your IR detector. Another alternative is to use a comparator whose input range includes the positive rail.

 

[Chrisd]

Many thanks for your help - Will implement the changes and see if the problem leaves. What is really interesting is the circuit came via a friend and is apparently in use as a paper-out detector or similar -which makes me wonder how reliable that product is!

Once again - thanks for your help

 

[Roff]

I hope this solves your problem. The reason the LM393 will not work with high input common mode voltage is that is has a PNP differential input stage, and each input also has a PNP emitter follower. This means that the input threshold voltage (from your voltage divider) has to be low enough to allow two series PN junctions to turn on, in addition to allowing some headroom to allow at least a couple of tenths of a volt for the tail current source of the differential pair. This adds up to, as I said, around 1.5v. BTW, this spec goes to (Vcc-2v) at 0 deg C.
I also wonder about the reliability of the printer that you mentioned. PN junctions have a negative temperature coefficient (~ -2mv/deg C), so as long as the comparator stays warm, the VBE drops will be relative low, and less headroom is required.
I suspect that you could buy some cold spray and hit the comparator with it and it would fail reliably. :roll: I can't explain why it worked when you took it out of the freezer.
BTW, another potential fix is to power the LM393 from a higher supply voltage - but you probably don't have one.

 

[Chrisd]

Thanks Ron Will have an altered one and a std one in operation in the next couple of days and will eargerly watch.