Help, H11L1 opto

[muso52]

Hi Guys,

Having struggled over the last few days to get what is my first project of the ground, have come to the conclusion I need your help!!

The circuit I am trying to build is a simple analog to digital convertor from a variable dc voltage, nominally 4 – 18v. Basically I am using a multi- diode ladder array to pull of voltage triggers (inputs) at differing voltages. i.e If the input supply voltage does not overcome the V drop (at differing) levels the input drops out. I should say that this will be interfaced to TTL logic and hence to a computer. With me so far, good! Initially the idea worked using a standard AN425 opto- isolator( to decouple DC voltage ground from Pc).However as the voltage was variable (Irrespective of the diode ladder) in- determinate states were experienced causing a rethink. The use of a Schmitt trigger was thus used to ‘ square up’ the pulse which is ok. One problem to overcome is in the use of a zener diode ( to hold voltage at a set level ) . Obviously, as the input voltage to the zener (prior to the opto) is variable with the dc voltage, a way to regulate the current is required thus the use of a LM 334 Constant current device to source the opto at these differing voltages. I hope you are following this!! Now he problem I have is that I decided to try the H11L1 schmitt opto isolator to replace the an425 and subsequent Schmitt trigger. Simpler, but no!! Can I get the h11l1 to work,no! I even set it up as per Isocoms pdf as a test switch all to no avail. All voltages correct Vcc Vd and currents Ic max etc,no good. Ok I initially thought it was a duffer and having purchased ten of them tried another, still no joy. The output pin (4) rises to Vcc and stays there irrespective of input. Any ideas you guys??

I can hear you all say that a A to D device might well be simpler , however have not managed to source one that fits the bill. I hope therefore that maybe you guys can help and I would really apprciate your input, Thanks in advance,

 

[ericgibbs]

hi,

Ive had a quick look at the datasheet.
Can you post a sketch on how you have connected the opto [ in that pdf test mode]

Use the 'Manage Attachments' button lower down the Reply page to post the sketch.

 

[ericgibbs]

hi,
This is from the data sheet, RL is 270R.

 

[muso52]

Yep, thats the Circuit. If I remember (from last night) (getting on a bit!) sourced the input at 7ma. RL= Vcc -Vd / 7ma. Id measured with Fluke.
That Cct with values as suggested (by Isocom) and Nothing! Current flowing through input (pins 1,2) so can only assumed not switching. But more than one Duffer in a batch of ten from RS?

 

[audioguru]

When the LED turns on then the output is supposed to go low.
The LED is at pins 1 and 2. If you don't limit its current with a series current-limiting resistor then the LED will instantly blow up.

Rhe output needs a load resistor to the positive supply. If the output is connectrd directly to the positive supply then it will blow up instantly when it tries to go low.

 

[ericgibbs]

Yep, thats the Circuit. If I remember (from last night) (getting on a bit!) sourced the input at 7ma. RL= Vcc -Vd / 7ma. Id measured with Fluke.
That Cct with values as suggested (by Isocom) and Nothing! Current flowing through input (pins 1,2) so can only assumed not switching. But more than one Duffer in a batch of ten from RS?

hi,
Are you saying the test circuit is working.?

Obvious point, have you checked the opto's model number printed on the ic to make sure the ic's are what you think they are.?
I have had the wrong ic type in a correctly marked package, at times in the past.

Can you please post your circuit so that we can see what maybe wrong.

 

[muso52]

Hi Guys,

Apoligies for not coming back sooner but with the sun out down these parts drove down to Padstowe for the day with the Lady Wife!

Back to the H11L1

Audioguru. Fully understand all you say with respect to the input across the H11L1 (pins 1 and 2) as I said in first post, eventually will put LM334 constant current device on the input to the H11L1 because of the varying voltage applied.

Eric. With my failure to get the H11L1 working I connected it up as per the test switch circuit you posted on an earlier response This being the one from the data sheet (pdf) of the device. All voltages, resistor values as stated for the output side i.e 5v to pin 6 with RL 270r across pins 6 and 4. Looking back at my second post I stated that RL = Vcc –Vd / 7ma . My mistake. This would imply that the R was the load resistor across pins 4 and 6. It is of course the led dropping resistor( Re) for the input.
With respect Eric, I see little point in sketching out the circuit as it is basically the test switch circuit from the data sheet with a common VCC (5v) and ground and yes it is not working!! There is obviously something elementary I am missing because 3 H11L1s have been tried with no joy. As I said earlier, with a fluke between pin 2 and ground I can see 7ma so can assume that the input led is working UNLESS I have got the values wrong somewhere , but I do not think so. So to reiterate , with the device connected as suggested in the data pdf, for me, IT DON’T WORK!!!

My understanding of the device is that assuming current is flowing through the input the output should go low. This ‘low’ when measured at pin 4 will of course be Vcc minus the volt drop across the load resistor and I would expect to see about 1v this still being a Digital Low and also the voltdrop I would expect across the internal Switch (Transistor). The whole idea behind using this device was that with a traditional opto, indeterminate states were experienced because of the varying voltage input. Hence I am still hoping that I can use this device to give me two state outputs from a varying voltage input. If I can not get it to work then it is a trusty opto coupled to a Schmitt trigger. Finally Eric, how about you work out a value of Re ( the input led current limit) at a Vcc of 5v and I will drop that in . Maybe my understanding of the data sheet is wrong!!

Many thanks for your help and interest. Bill, Plymouth

 

[ericgibbs]

hi Bill,

Can you try the follow test to the opto.

Connect a 270R between +5V and the opto emitter anode and the opto cathode to 0V.
This should forward bias the emitter.
Using your voltmeter, measure the voltage across the emitter, it should about 1.2v to 1.5V.

If you get a zero voltage across the opto, measure the current thru the 270R.
If you measure a current of about 20mA the emitter is short circuit.

If you get +5V across the emitter it could be o/c emitter.

If the opto emitter checks out OK, we will then look at the detector.

 

[muso52]

Hi Eric,

Voltage on pin 1 (after 270r) = 1.221v as expected. However this wasnt untill I dropped in my fourth H11L1, the previous one appears open CCt emitter as 5v seen after 270r resistor. No joy on the output side yet thou.
bill

 

[muso52]

Again Hi Eric,

Well out of a batch of Ten (RS MIn Quantity) It would appear I have Five Duffers so far, difficult to believe!! Will re check them all and the remaining! Have now at last got one to work! Using RL (load res) of 270r.
Rs are pretty good though, will contact them tomorrow and they normally just despatch replacements.
If I may seek your advice further I would appreciate it. It seems perfectly feasible to place an LED in series with a suitable load resistor between pins 6 and four. Why? For a visual indication of the triggering of the schmitt. You will recall I stated that I will eventually source the input to the device from a variable voltage. I therefore would like a visual indication of the triggeriing i.e. state change when the input voltage is raised from zero to a point when the input led conducts and hence the triggering of the schmitt. I want to see the led on/off and not change in light output . Basically a visual indication of the
triggering characteristics of the schmitt. I have to have a TTL compatible input as indeterminate states are a no no on the input to the PC and software!!

With thanks, Bill (for one day only) sunny Plymouth.