AC input to TTL problem

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Klif_dor

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Hi all,

I'm new to these forums but I have a problem that is stumping me.

I have a circuit with 0 and 5 volt rails and a periodic input in the 400 Hz range with amplitude of +/- 1 Volt to 200mV.

I was thinking of using an optocoupler 814 as a buffer to avoid negatively biasing my 5 volt logic circuit. From the spec sheets for the 814 I found the following circuit to adapt AC signals to a TTL circuit. However, I'm measuring a square wave output of the attached circuit with peak voltage of 2.6V and low of -2.4V. The square wave distortion is not a problem since the frequency is preserved. I'm desiring a square wave from 0 - 5 Volts.

am I not measuring the output correctly or is the voltage still swinging negative. Can anyone point me in the right direction?

thanks in advance,
Kliff
 

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hi,
The Vfwd drop of the emitter diodes is 1.2V at 20mA, the current transfer ratio is from 20 to 300%.

You say your drive signal is only with amplitude of +/- 1 Volt to 200mV.

With a +Vcc to the output resistor you have a signal of peak voltage of 2.6V and low of -2.4V.

This should not happen, check the opto connectors and the +Vcc supply and the way are measuring the output.
Is the output connected to anything else when you are measuring??
 
I used a function generator between pins 1 and 2, a 5 volt pwr supply for Vcc and Ground and the scope is hooked between pins 3 and 4. I tried the circuit with and without a load... same results if I recall.
 
Disconnect the input to the opto-coupler. Adjust your probe locations until you can measure VCC between the collector of the transistor and the emitter of the transistor which should be ground. Make sure the scope is DC coupled and not connected to the ground of the generator.
 
Thanks for your help, I think it was the DC coupling, I'm able to get an output from 30 mV to 4.95 Volts... nice and TTL friendly,
 
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