![\"\"](\"http:\/\/www.retro.co.za\/blog\/wp-content\/uploads\/2022\/10\/LogicProbe.jpg\")
<\/a><\/p>\nI built this logic probe a … long … time … ago. Late seventies, I think. You can see where I cut it down the middle to fit in a pill bottle, but I straightened it again later.<\/p>\n
And I could lay my hands on it when I needed to, a week ago. Yea, I don’t throw stuff away. Especially not useful stuff, and a logic probe counts as useful. I had to replace the 7400 which is now in a socket, back when I built it a socket would have been a luxury.<\/p>\n
Above schematic from stackexchance.\u00a0 And when we built one for Z-80 boy the other day, we found that R2 should indeed be 1k, because otherwise it doesn’t work right.<\/p>\n I also added a diode for reverse-voltage protection which is not a bad idea.<\/p>\n There are of course many other, better designs out there (electronicsforu.com<\/a>,\u00a0 elechelp.com<\/a>, circuitfee<\/a>…), including one in Everyday Electronics of September 1980<\/a>*. But this one is simple and it works.<\/p>\n * I “subscribed” (in that my parents would give me the money every month and I would walk down to the CNA on Voortrekker Road and buy a copy, which they reserved for me) to Everyday Electronics from September 1978, or at least, that’s the earliest cover I remember.<\/p>\n","protected":false},"excerpt":{"rendered":" There’s a kid around the corner who is building a Z-80 computer on veroboard. This is not his story. But in helping him I realised he needed a logic probe. This is that story. I built this logic probe a … long … time … ago. Late seventies, I think. You can see where I cut it down the middle to fit in a pill bottle, but I straightened it again later. And I could lay my hands on it when I needed to, a week ago. Yea, I don’t throw stuff away. Especially not useful stuff, and a logic...<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[66,75],"tags":[],"class_list":["post-5144","post","type-post","status-publish","format-standard","hentry","category-computers","category-electronics"],"_links":{"self":[{"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5144","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5144"}],"version-history":[{"count":5,"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5144\/revisions"}],"predecessor-version":[{"id":5614,"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5144\/revisions\/5614"}],"wp:attachment":[{"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5144"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5144"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.retro.co.za\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5144"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}<\/a><\/p>\nI have no idea where I got the original schematic from, but this is it, except that my<\/del> Edit (2024): Well, turns out I keep good records when I remember where to find them, I got the original schematic from Practical Electronics, March 1983<\/a>, it was submitted by G. Coleman from Rochester in Kent, with R2 = 1k and C1 and C2 = 1u in his schematic, same as my build.<\/p>\n