# logic probe

Discussion in 'General Electronics Discussion' started by foTONICS, Jun 6, 2012.

1. ### foTONICS

332
9
Sep 30, 2011
so I just want a basic logic probe, red = LOW & green = HIGH.

I don't care for frequencies just those two basic needs although I am putting some rules down for my project. Mainly when I'm not touching anything I don't want either of the two LED's to turn on.

Attached is my idea. The probe input would be between R2 and R3. When a logic level high is put on the input the potential will be the same for the top two resisters and no current would flow right? I applied the same idea to the bottom two resistors when the probe input is grounded.

I just can't wrap my head around how to get the top gate not to trigger when the probe isn't touching anything, any ideas?

I would like to keep away from anything too bulky as I'm gonna cram this inside a pen

thanks for the help all!

*** i just realized i attached the one input of both the and gates to ground, that should be to the other side of the 5V source ****

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2. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Don't use TTL. They have a low input resistance that will upset the voltages you're trying to set up in your resistor chain.
A simple way to do what you want is to use a dual comparator such as the LM393. When the input voltage is above the HIGH threhsold, one comparator activates and the red LED lights. When the input voltage is below the LOW threshold, the other comparator activates and the green LED lights.
You bias the probe (input) at a voltage that's in between the low and high thresholds, so when the probe isn't touching anything, neither LED lights. You use a resistor voltage divider for this.
I'll draw up a schematic and post it soon.

3. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Simple logic probe using LM393

Here's a simple logic probe using an LM393 dual comparator.
R1 and R2 bias the probe to a voltage half way between the HIGH and LOW thresholds of the logic family that you're working with.
In this design I've assumed thresholds of 2/3 VCC and 1/3 VCC so the probe is biased at 1/2 VCC which is in between the thresholds. This ensures that both LEDs are extinguished when the probe is not connected to anything.
The thresholds are selected by R3, R4 and R5. You can adjust these resistors as necessary, according to the logic family you're using, and adjust R1 and R2 so the probe's idle voltage is about half way between the two thresholds.
You can also change the input resistance of the probe by varying the absolute (rather than relative) values of R1 and R2.
The comparators have open collector outputs so you connect the LEDs to the positive supply rail.
D3 protects the probe against reverse power connection, and D1 and D2 protect it against over/undervoltage at the probe. Actually, a small value series resistor in series with the probe connection should be added.

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4. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Here's the same design with more sensible resistor values and an input protection resistor. The input resistance is 35 kilohms. The thresholds are the same as before.
BTW I noticed you wanted green for H and red for L. Logic probes normally use the opposite convention.

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5. ### foTONICS

332
9
Sep 30, 2011
so with nothing touching the probe tip the negative input of UA1 is at 2.5V

If the tip touches a logic high of 5V what does this do to the reference voltage of UA1, does it add to it?

I've tried searching these schematics online before but get confused when I start thinking of what voltage each input sees when the probe touches something.

6. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Yes.
No. U1A is just a comparator; it compares the voltage at its two inputs. The voltage at its non-inverting (+) input is set by the three-resistor voltage divider, and does not change when the probe is touched to some voltage.
When the probe is touched to a logic HIGH voltage, U1A's inverting (-) input goes to 5V but the non-inverting (+) input remains at 2/3 VCC (i.e. 3.333V) and U1A's output goes low, illuminating the red LED.
The inputs that come from the three-resistor voltage divider are at fixed voltages (fixed percentages of the supply rail). The other two inputs follow the probe voltage.

7. ### foTONICS

332
9
Sep 30, 2011
Would a LM741 be a suitable replacement? It's the only op amp I have on hand

8. ### foTONICS

332
9
Sep 30, 2011
C1 is a smoothing cap right?

9. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
No, the 741's input voltage range doesn't go close enough to the supply rails. Even the LM393 is getting close to its limit at 5V supply and past its limit at 3.3V supply. Yes, C1 is just for smoothing.

10. ### foTONICS

332
9
Sep 30, 2011
built circuit

Hey man thanks a ton for the help,

I've attached some photos of the circuit as well as some operating pics of the hi and low states.

You mentioned not to use the 741 but I had already built the circuit at that point. it seems to be working correctly but am I endangering the circuit by using a 741?

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11. ### foTONICS

332
9
Sep 30, 2011
to get the cct working correctly I had to add a third wire as a ground so I could properly bias the reference terminals

12. ### foTONICS

332
9
Sep 30, 2011
i see what you mean now about the supply rail voltage. I reworked the biasing so I could dump the third ground wire and now the low won't trigger, the output stays off regardless of it's input

13. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Is the circuit now exactly the same as what I posted, apart from the 741s instead of the LM393? If so, and the LOW LED doesn't work, it's probably because of the input range of the 741. The LM393 is pretty widely available, I think. You really ought to get one.

14. ### foTONICS

332
9
Sep 30, 2011
The only thing that changed from the schematic you posted was how I connected the power. I had to use a dual supply in series mode to get the 741's to work. I gave them +/- 5V but this required me to have my biasing resistors connect from the +5V to ground instead of the -5V. This required me to add a third ground wire in order for both of the LED's to stay off.

After I got that working I tried to get rid of the ground wire by using +/- 2.5V, but that's when I ran into the supply rail problem you mentioned earlier.

In order for this thing to be less bulky it looks like I'll have to go pick up some of those IC's you were talking about

Thanks again for all the help, looks like I was just making this more complicated than it actually was!

15. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
You're welcome Good luck!