The external base on a 3-pin photodarlington. What to do with it.

Hello,

this question is about the external base lead on the 3-pin photodarlington packages, and what to do with them.

I bought some CL138 photodarlingtons on ebay. Googling 'CL138 datasheet' returned very little of use, the datasheet is mostly white space.

NTE3036 looks like an equivalent, and on its datasheet it states 'external base for added control.' Initially all was well, but when I took my contraption outside to calibrate the light sensitivity I got some unreliable behaviour from the photodarlington. I suspect it's because I haven't dealt with the external base lead properly - I just left it floating - some stray current was entering the base perhaps, which of course is bad news given the large gains of these devices. But I'm not qualified to say.

Searching on this forum returned a thread about optocouplers with external bases (specifically about using them in their linear regime to send analogue signals.) Someone suggested the base can be connected to the Emittor via a 100K resistor. Someone else suggested that it might be best to have the base at a lower voltage than the Emittor(!?)

All I want to do is use the photodarlington as a switch (in line with my camera's wired shutter release, so it only works during the day) . I don't want an external base for 'added control.' The 100K resistor suggestion seems to work, but I'd like to know for sure, since this thing has a habit of working regardless of what I do with the base, but then suddenly behaving badly for no apparent reason.

Any advice, explanation, discussion would be of interest. Seems like such a trivial problem.

Thanks in advance,
AR.

I would connect it to the negative of the supply via a 2k2 resistor.

Same as for a pull up or pull down resistor on TTL circuits.

astronomerroyal said:

since this thing has a habit of working regardless of what I do with the base, but then suddenly behaving badly for no apparent reason.
AR.

Click to expand...

What do you mean by "behaving badly"? You must be firm and specific or else any punishment you dish out will be met with confusion.
A resistor between the base and the emitter (Or to ground from the base) will reduce the sensitivity of the photo transistor. You can adjust this resistor to set the circuits sensitivity to light. No resistor means maximum sensitivity and 0hm: means zero sensitivity.
If you are trying to detect a flash bulb going off, an inductor between the base and emitter will make the transistor very insensitive to ambient light but quite sensitive to the fast rise time of a flash pulse.

RODALCO said:

I would connect it to the negative of the supply via a 2k2 resistor.

Same as for a pull up or pull down resistor on TTL circuits.

Click to expand...

Since a Darlington requires Vbe ~ 1.2V to turn on, it seems to me that 2.2k would require around 1.2V/2.2k ~ 500uA of photocurrent just to overcome the 2.2k. Doesn't this defeat the purpose of using a Darlington?
Speaking from no experience, the tradeoff is between sensitivity and turn-off time. The base-emitter resistor will speed the discharge of capacitance and the excess base charge resulting from saturation, at the expense of sensitivity to light.

All I want to do is use the photodarlington as a switch (in line with my camera's wired shutter release, so it only works during the day) . I don't want an external base for 'added control.' The 100K resistor suggestion seems to work, but I'd like to know for sure, since this thing has a habit of working regardless of what I do with the base, but then suddenly behaving badly for no apparent reason.

Click to expand...

hi AR,
Do you have a sketch to post, showing how the opto is connected to the shutter release.?

Here's how I've wired up the photodarlington to my camera's shutter release.
Sorry, don't know how to draw circuit diagrams.

camera +V ------------ \ (manual switch)--------- C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |----B
camera GND ---------------------------------------- E

(ignore the periods) To be clear, C=collector, E=emittor, B=base. Manual switch is just the shutter button one presses. Once pressed the Collector sees +V.

By 'behaving badly' I mean that 'the photodarlington is acting like a closed switch even in total darkness', from which I infer that a rogue, 'sufficiently large' current is entering the external base wire such that the darlington conducts.

The possibility that I can use the external base to calibrate the sensitivity of the darlington is intriguing. I hadn't thought of that, nor had I encountered it in any online searches. I had previously set the photodarlington's sensitivity by placing layers of paper over the sensor, but I understand (in a rudimentary way) how adding a potentiometer to the base might affect the base current entering the darlington and thus set the sensitivity.

If I connected the base to ground would the current induced by the light simply travel to ground and not 'turn on' the transistor?

I have noticed that - while it was 'well-behaved' - the camera could be fired *once more* after the lights had been turned off. It was as though there was some capacitance at the base. This was with the base connected to GND with a 10K resistor. When it was persistently behaving badly the air was very humid, I thought perhaps that was the cause.

I'd like to understand How Best to use a Photodarlington. I do have some standard phototransistors, but photodarlingtons seem interesting. Actually I don't need great light sensitivity, nor do I require fast switching times, but I'd still like to understand the photodarlingtons.

I appreciate your contributions, thanks.

hi,
Depending upon the source resistance of the Camera's +V to the normal push switch to 0V, you could be exceeding the voltage and/or current rating of the opto transistor.

Normally a opto would would have a resistor in the collector circuit and the output to the following circuits taken from the collector.

I wouls suggest you remove the opto, place a voltmeter in place of the opto, measure the voltage when the switch is pressed and also the current flowing thru the meter.

Lets know what you measure.

astronomerroyal said:

Hello,

this question is about the external base lead on the 3-pin photodarlington packages, and what to do with them.

I bought some CL138 photodarlingtons on ebay. Googling 'CL138 datasheet' returned very little of use, the datasheet is mostly white space.

.

Click to expand...

I tried pulling up the CL138 on alldatasheets.com and got no returns. You sure about that part number?

AllVol said:

I tried pulling up the CL138 on alldatasheets.com and got no returns. You sure about that part number?

Click to expand...

Sorry about the quality but I've attached two photos; one of the datasheet I did find and a fuzzy close-up of the actual device. It's a small metal can with a transparent window on top. Two microscopic dots I assume are the photodectectors and the larger two dots seem rather bizarrely to be the legs of the E and B emerging through the top. I just found the CL138 datasheet on my computer, that's now attached also.

Made by Micro electronics Ltd (MEHK)
38 hung To Rd, Kwun Tong
Kowloon, Hong Kong.

I'll have to drop by.

Eric: Generally the camera's voltage is about 3V. I think current is only passed momentarily, although I'm not sure of its magnitude I am fairly sure that it's very small - this is a dSLR. I don't know what's inside but I would imagine the remote shutter button simply switches a transistor on/off.

Incidentally, how would I measure a transient current if I don't have a known load across which to measure the voltage? I have access to an oscilloscope and multimeter, though the multimeter is slow

thanks