LP-425 Logic Pulser
I watched an 8-Bit Guy youtube video in which he used a logic pulser in one hand and a logic probe in the other hand to test some TTL chips. After this, I wanted to find something similar to use myself. Handheld logic probes are not too difficult to find, although it can be challening from the reviews and specs to figure out which one to buy. I had a lot more trouble, however, trying to find a logic pulser.
I ended up buying an Elenco LP-425 Logic Pulser kit. I was rather suprised though when I got it: I had assumed "kit" meant the pulser plus some cables, but actually it was a DIY kit with around 50 or 60 components.
Here are some pictures of the unit after soldering and assembly:
- Includes a 10 page Assembly and Instruction manual, of good quality, including schematics as well as a troubleshooting/repair guide if the unit doesn't work after you finish building it.
- Includes 400PPS and 0.5PPS frequencies. The tip gives an impulse signal, but square wave and sync outputs are available also out the posts.
- After troubleshooting one initial problem, it seems to be working fine for me.
- The board uses really large pad sizes. Normally large pads are a good thing, but they are so large that it is not really possible to fill up the whole pad without globbing on too much solder. So, most pads end up half-filled with solder which looks rather unprofessional in the end.
- Huge rod shaped pins are included, to serve as posts for the SQ, SYNC, and GND signals. It is really hard to get these soldered onto the board without damaging the pads or getting the rods on crooked.
- The power cable included in the kit does not match what is described in the manual. The power cable described in the manual is supposed to have one ribbed wire. Also the included power cable is too thick to fit through the holes on the board.
- The alligator clips included are a poor design which is difficult to get properly soldered and crimped onto the cable.
- If you don't get the front label on perfectly, this causes some challenges. I wasted a lot of time troubleshooting a problem with the output signals, and in the end realized that the label was not allowing the switch to move completely over its full range. So I had to cut the label in a few places.
Here are the waveforms on a scope. First is the raw signal, but to see the 10ms tip pulse width you need for the tip to be part of a resistor network. What is shown below matches what is shown in the manual.
0.5 PPS (no load)
0.5 PPS tip signal
0.5 PPS square signal
0.5 PPS sync signal
400 PPS (no load)
400 PPS tip signal
400 PPS square signal
400 PPS sync signal
0.5 PPS (between two 1k resistors)
0.5 PPS tip signal in resistor network
0.5 PPS tip signal in resistor network (Close-up)
400 PPS (between two 1k resistors)
400 PPS tip signal in resistor network
400 PPS tip signal in resistor network (Close-up)
Probes and pulsers are very handy if you are working on old boards. My personal favorite tool is the huntron, but those pulsers are great to force a result.