ttl Voltage comparator for +/- 7V - Electronic Circuits Projects Diagrams Free

dwurmfeld · 17th January 2008, 10:35 PM

I need to detect the presence (with a TTL logic level output) of two separate power supply voltages, +7V and -7V. Is there a dual comparator IC I can use that will give me a TTL output for a negative/positive comparison?

What would the input circuit look like for this? Would a LM311 be a good choice? Would you pull up the output of the comparator to +5V?

The only thing I know about analog is: "Ground is where you grow Roses!"

Thanks in advance for the help.

David

Papabravo · 18th January 2008, 03:25 AM

The LM311 can be cranky and difficult to stabilize. I would suggest the LM393 Dual or the LM339 Quad. Make sure that any circuit you design observes the requirement that the input signals be within the common mode range of the device. You can satisfy the requirement by powering the comparators from +9V and -9V to measure values in the range [-7V .. +7V]

Roff · 18th January 2008, 04:52 AM

Here ya go. Connect the ground pins from your +/-7V supplies, and from the 5V logic supply, to the ground (the little triangles) on this circuit.

dwurmfeld · 18th January 2008, 03:00 PM

Thanks for the quick reply, I noticed in Ron's schematic that the power to the LM339 is single ended to +5V, am I reading that correctly, or do I need to power it with +/- 9V as "PapaBravo" suggests? Again, thanks for the circuit, I'll try it today.

David

Roff · 18th January 2008, 06:05 PM

Quote:

Originally Posted by dwurmfeld

Thanks for the quick reply, I noticed in Ron's schematic that the power to the LM339 is single ended to +5V, am I reading that correctly, or do I need to power it with +/- 9V as "PapaBravo" suggests? Again, thanks for the circuit, I'll try it today.

David

You can use a single +5V supply, because R6 and R8 level shift the -7V input into the common mode range of the comparator.

dwurmfeld · 21st January 2008, 10:09 PM

Hello!

I tried the circuit, and have the following results:

+7V - on at 3.6V, off at 4.0V
-7V - on at 2.9V, off at 3.3V

Any ideas on how to troubleshoot the circuit?

Thanks.

David

eng1 · 21st January 2008, 10:49 PM

Quote:

Originally Posted by dwurmfeld

I tried the circuit, and have the following results:

+7V - on at 3.6V, off at 4.0V
-7V - on at 2.9V, off at 3.3V

Any ideas on how to troubleshoot the circuit?

That's fine if you need to check the presence of ±7 V. BTW did you mean
-7V - on at -2.9V, off at -3.3V ?

If the supply voltages are present, the outputs of the comparators are high (+5V). If they're not present, R8 and R7 force the output low.

Placing the threshold voltages at about 3 or 4 V with hysteresis gives you some margin for errors caused by tolerances+noise.

dwurmfeld · 21st January 2008, 10:55 PM

You are correct of course, I was not looking at it properly. I understand the circuit now, and I indeed need it only to detect the presense (or absense) of +/-7 volts.

Thanks again for the great circuit idea.

David

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17th January 2008, 10:35 PM	(permalink)
dwurmfeld	ttl Voltage comparator for +/- 7V I need to detect the presence (with a TTL logic level output) of two separate power supply voltages, +7V and -7V. Is there a dual comparator IC I can use that will give me a TTL output for a negative/positive comparison? What would the input circuit look like for this? Would a LM311 be a good choice? Would you pull up the output of the comparator to +5V? The only thing I know about analog is: "Ground is where you grow Roses!" Thanks in advance for the help. David __________________ Save the Earth - its the only one with Chocolate!

18th January 2008, 03:25 AM	(permalink)
Papabravo	The LM311 can be cranky and difficult to stabilize. I would suggest the LM393 Dual or the LM339 Quad. Make sure that any circuit you design observes the requirement that the input signals be within the common mode range of the device. You can satisfy the requirement by powering the comparators from +9V and -9V to measure values in the range [-7V .. +7V] __________________ We never have time to do it right; but we always have time to do it over.

18th January 2008, 03:00 PM	(permalink)
dwurmfeld	Thanks for the quick reply, I noticed in Ron's schematic that the power to the LM339 is single ended to +5V, am I reading that correctly, or do I need to power it with +/- 9V as "PapaBravo" suggests? Again, thanks for the circuit, I'll try it today. David __________________ Save the Earth - its the only one with Chocolate!

21st January 2008, 10:09 PM	(permalink)
dwurmfeld	Hello! I tried the circuit, and have the following results: +7V - on at 3.6V, off at 4.0V -7V - on at 2.9V, off at 3.3V Any ideas on how to troubleshoot the circuit? Thanks. David __________________ Save the Earth - its the only one with Chocolate!

21st January 2008, 10:55 PM	(permalink)
dwurmfeld	You are correct of course, I was not looking at it properly. I understand the circuit now, and I indeed need it only to detect the presense (or absense) of +/-7 volts. Thanks again for the great circuit idea. David __________________ Save the Earth - its the only one with Chocolate!