Hello Guys,
Not so much a newbie here, oldbie would be more appropriate!
What I thought would be relatively simple to build has confronted me with a number of difficulties and so I would appreciate your help and advise.
The requirement of the circuit is to ‘sense’ when a varying and predetermined DC voltage is applied to a circuit and convert this to a TTL output.
Essentially the above comprises two elements.
The first one is a standard variable (via pot) 0-14v 1a DC motor controller using a Darlington-pair driving an Output Transistor.
The second element comprises a circuit that senses when a particular voltage, say 6v (and above) is outputted from the controller and changes a logic 0 to 1 i.e. 5v
It should be stated also, that the former needs to be electrically isolated from the dc motor output.
Now the idea I came up with was the simple use of a diode ladder network.
Depending on the collective voltage drop of the diodes and the physical connection point on the ladder where I ‘tapped’ a feed of to a opto-Schmitt, I thought this would work as follows. With say, a given and known output of 6v from the motor controller and with four diodes in series, this would drop the voltage by 2.8v (4 x .7v) leaving 3.2v to trigger the photo in the opto device. Now the opto-schmitt had a CCD (LM334) as a constant current source, biasing a suitable Zener to the input of it. This appeared to work well initially. With a led connected to the opto output side for a visual indication, this simple circuit when inserted at different points in the ladder, would trigger dependent on the applied voltage. However despite the use of a opto-schmitt isolator, a clear change in ttl output is not forthcoming for the following reason.Unless the phototransistor is fully saturated, i.e. fully on, an indeterminate level exists which is a no no as you can imagine
So, has anyone any other ideas how I could implement the above in a different circuit maybe?
Would really appreciate your input.
Muso 52
Not so much a newbie here, oldbie would be more appropriate!
What I thought would be relatively simple to build has confronted me with a number of difficulties and so I would appreciate your help and advise.
The requirement of the circuit is to ‘sense’ when a varying and predetermined DC voltage is applied to a circuit and convert this to a TTL output.
Essentially the above comprises two elements.
The first one is a standard variable (via pot) 0-14v 1a DC motor controller using a Darlington-pair driving an Output Transistor.
The second element comprises a circuit that senses when a particular voltage, say 6v (and above) is outputted from the controller and changes a logic 0 to 1 i.e. 5v
It should be stated also, that the former needs to be electrically isolated from the dc motor output.
Now the idea I came up with was the simple use of a diode ladder network.
Depending on the collective voltage drop of the diodes and the physical connection point on the ladder where I ‘tapped’ a feed of to a opto-Schmitt, I thought this would work as follows. With say, a given and known output of 6v from the motor controller and with four diodes in series, this would drop the voltage by 2.8v (4 x .7v) leaving 3.2v to trigger the photo in the opto device. Now the opto-schmitt had a CCD (LM334) as a constant current source, biasing a suitable Zener to the input of it. This appeared to work well initially. With a led connected to the opto output side for a visual indication, this simple circuit when inserted at different points in the ladder, would trigger dependent on the applied voltage. However despite the use of a opto-schmitt isolator, a clear change in ttl output is not forthcoming for the following reason.Unless the phototransistor is fully saturated, i.e. fully on, an indeterminate level exists which is a no no as you can imagine
So, has anyone any other ideas how I could implement the above in a different circuit maybe?
Would really appreciate your input.
Muso 52
