R6 does it. Because of the resistor in the emitter, the Darlington acts as a constant current source.Originally Posted by Roff
Replacing an NPN for a PNP in the following circuit?
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23rd March 2011 11:21 PM #11
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24th March 2011 01:28 AM #12MikeMl
0Mike ML.
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24th March 2011 02:31 AM #13Roff
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26th March 2011 04:02 PM Thread Starter #14Delinquent
Mike, I couldn't get a couple of parts - closest fit were a 470k pot and for the scr a x00619, bit having no joy getting it working. Do you think just those two components are ok in your circuit? I tried adding a simple led with a thyristor to show when there was input from the pt, but not even that lit!
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27th March 2011 02:49 PM #15MikeMl
Do you have a DMM that has an input impedance > 10MegΩ in the DC Volts mode? If so, measure the voltage at the node named "det" in the circuit (wrt ground) while putting/removing an object in the light path. Report back with the two voltages.
0Mike ML.
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27th March 2011 05:16 PM Thread Starter #16Delinquent
Thanks Mike. With object in path voltage was 0.94v, with the path clear it's 1.85v.
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27th March 2011 11:38 PM #17MikeMl
That means that very little energy from the photo-emitter is reaching the photo-detector. What is the emitter? How hard are you driving it? What is the distance from the emitter to the detector? What kind of ambient lighting? Originally Posted by Delinquent
0Mike ML.
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28th March 2011 08:22 AM Thread Starter #18Delinquent
The emitter is a TSUS5400, which was suggested by the vendor as spectrally matched to the detector. Distance that that measurement was taken at was very small, only about 2 inches at most!!! Ambient light was 3 low energy lightbulb, each equivalent to 60w
For a full picture, the sensor rig (wired as per the diagram on prev page) is two 3ft lengths of a medium quality speaker cable, one end is a 3.5mm stereo jack. The 470o resistor is at the emitter end of the cable, the two +ve's are joined at the stereo jack.
The funny thing is, I have had problems with pretty much every circuit I've tried off the internet, I've made lots of sensors up in the past with different lengths and qualities of wire, different emitters and detectors... I ended up finding a simple detector circuit on the net and made up a simple dedicated unit to test the leads involving a thyristor a 220 ohm resistor and an LED. All the leads I have, tested, work well on the unit. All the sensors I've made light the LED (albeit dimly) at a distance of about 2ft, but as soon as I plug it into a trigger circuit it seems to drop to a matter of an inch or so!0Last edited by Delinquent; 28th March 2011 at 08:30 AM. Reason: missed a word out!
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28th March 2011 01:53 PM #19MikeMl
You did try to "align" the optical axis of the emitter with that of the detector?
Try it while watching the voltmeter as above.0Mike ML.
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28th March 2011 02:09 PM Thread Starter #20Delinquent
indeed, I checked all that when originally making the leads up and consequently when I've run into problems, it was what prompted me to make the little test unit. The rx has quite a wide viewing angle and testing it with a mm showed the axis makes little difference at close range.
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