# TTL logic probe ( 5V dependent ok )

Aug 29, 2011
5
Hello everyone. this is my first post here.
I want to make a logic probe for 5 volt ttl systems.
I thought it'd be cool to make one i could put in a pen.
i see a lot of schematics that would fit in a pen using resistors and op-amps.
the thing is they don't really seem to be specific to ttl logic levels.

would it be practical to build one (hopefully that could fit in a pen cause i think that'd be cool) that would have one led for low ( 0-0.8V) one for undefined ( 0.8-2V ) and one for high
( 2-5V)

id be thankful for any help i could get with this

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Depending on the size of your pen you may need to look at surface mount components.

Your voltage ranges sound fine, but you may want to add another indicator that detects pulses. That way if there's a low duty cycle repetitive pulse it will be obvious.

Aug 29, 2011
5
thanks for such a fast reply

okay so would some pulses be too fast to see on an led?
would using a two way two color led make it orange when pulsing?

if not what would it take to add in a light for fast pulses and how would i determine how long a pulse could be and still trigger the pulse light? also generally with 3mm leds how long in ms (within their rated voltage range) does it take to be visible?

would it be possible to make this work with just resistors and leds?

thanks for the help so far Steve

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
okay so would some pulses be too fast to see on an led?

Imagine a pulse that is several nanoseconds in duration...

would using a two way two color led make it orange when pulsing?

Not noticeably if the pulse duration was very short

if not what would it take to add in a light for fast pulses and how would i determine how long a pulse could be and still trigger the pulse light? also generally with 3mm leds how long in ms (within their rated voltage range) does it take to be visible?

You look for state transitions, and light the LED for say 1/10s each time you see one.

would it be possible to make this work with just resistors and leds?

Unlikely.

Here is a schematic and some explanation.

Aug 29, 2011
5
thanks for pointing me to that Steve.

over all this looks pretty good.

there are a couple things here which im not sure about though.

For one i can't tell what the cmos section is, unless its just a single pole, single throw switch that allows current to flow through R1 instead of it having to go through R4, R6 and R7
but the parts list calls for a single pole double throw as shown, does this matter either way??

also im not sure, but im figuring positive is positive power supply,and where it says earth is negative power. is the positive power also applied on the right where it says vcc?

the pasts list lists one LM358 IC, there appears to be two in the schematic.

and for the 14049 IC it is written twice on the schematic but i can't see exactly where it is.

sorry if i sound dumb I'm sure some of this would be obvious to someone who has experience with electronics and schematics, but i"m still very new to all this.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
For one i can't tell what the cmos section is, unless its just a single pole, single throw switch that allows current to flow through R1 instead of it having to go through R4, R6 and R7
but the parts list calls for a single pole double throw as shown, does this matter either way??

A SPST switch would do. The person making it possibly had a DPDT switch on hand, or maybe in an earlier version of the design it used more of the switch. It's hard to tell.

also im not sure, but im figuring positive is positive power supply,and where it says earth is negative power. is the positive power also applied on the right where it says vcc?
That's correct.

the pasts list lists one LM358 IC, there appears to be two in the schematic.
The LM358 contains 2 op-amps. So both of these are part of the same chip. Typically they would be labelled IC1a and IC1b to show this.

and for the 14049 IC it is written twice on the schematic but i can't see exactly where it is.
kind of the same as the LM358, the 4049 contains 6 inverters. Each inverter might typically be identified as IC2a .. IC2f (but they're so simple and small that it's not unusual not to do this).

Note that when you buy one of these, you're after a part generically known as a 4049, there are several manufacturers, and what you buy may end up being a SN4049, a CD4049, an MC14049 or one of many other variants with additional alphabet soup at the end.

sorry if i sound dumb I'm sure some of this would be obvious to someone who has experience with electronics and schematics, but i"m still very new to all this.
Don't worry, your questions are quite sensible.

I'm actually more concerned that you'd have to build this on a PCB to fit it inside a pen and that it may be well beyond your level of perseverance to get there.

It may be worth looking for other recommendations, especially of kits that already have the PCB made.

Aug 29, 2011
5
yeah i kinda started to figure that this would probably not be able to fit entirely in a pen.
i just got that idea cause i saw one that would but it was just resistors and leds that would basically read low a little under half the power supply and read high a little over half with no pulse indicator.

I still like the pen idea and what im thinking now, is i might try to find a small enclosure for most of the circuitry, and then have five wires running to a pen, one for the probe, one for the ground and one for each led. that way id have nothing but a stinger tip, 3 leds, 3 resistors and a little wire in a pen which i could do easily. do you think a regular s-video cable would work for this?

im just trying to think of a 5 wire (or 4 wire with ground) cable where a jack would fit into a pen.

Thanks again Steve. I really do appreciate your helping me out with this.

Replies
7
Views
1K
Replies
1
Views
1K
Replies
4
Views
2K
P
Replies
3
Views
2K
Sven Wilhelmsson
S
L
Replies
19
Views
4K
logjam
L