12V LED Test Light

Hi all I would like to make a basic led test light similar to the power probe but more basic(and cheaper). Basically, to have either two wires to the tester, one to positive and one to negative, a green led to show ground and a red to show hot or a single wire to the tester which when connected to neg and probe touched to positive lights a red led and when the same wire is connected to positive and probed negative, lights green. More than trying to get it made cheaper, I would like to put it together myself. Any schematic help would be much appreciated. Thanx

If you put a 10K ohm resistor in series with two diodes you should get what you are looking for. Make sure you wire the LEDs backwards from each other. With the LEDs wired backwards one will light up when you apply polarity one way and the other will light up when you reverse the polarity. My question to this though is why? You can buy a decent auto ranging meter from Craftsman for $25. It will do voltage, resistance, current, and if you find the right one frequency (great for remote start tach wires). Sure, you'll spend 10x the amount of money but you'll actually have device that will enable you to do 100x then a couple LEDs.Kevin Pierson

The disadvantage (or maybe advantage) with LEDs is that it will only pull up to 20mA; can only pull the source down to 2V; and only needs ~2V to illuminate. It will be difficult to judge by the LED illumination if the voltage is 2V, 8V, or 12V etc. OTOH, a 2W or 3W etc test light is ~160-250mA and can pull the source to GND (or +12V). Bulb illumination is far more proportional to voltage than a LED so judging 2V, 8V, 12V etc is much easier. The issue with the current draw is that many sources have an impedance - eg, an open circuit small bulb (2W - 7W) or a relay coil (60R - 160R => 200mA or 2.4W to 75mA or 0.9W. A 2-3W bulb (72R - 48R) will pull such sources down or up to ~6V (hence dim) whereas a LED (20mA or 600R) will essentially be fully illuminated. You could add a parallel 82R 2W resistor with the LEDs & their series resistor to duplicate test light behavior, or use a a 2W bulb instead. If using a 2W bulb as a dummy load for the LEDs, you could then omit the LEDs entirely. [ KP: :) ? ]

Thanks KP and Oldspark I do have a multimeter but I'm trying to get a bit involved in electronics and DIY, it makes the tool, when successful, worth much more than the monetary value. And my interest in electronics is all due to this forum and knowledgeable people like you. Thanks so much.

I was bore and made one using a glass fuse holder as a casing. I used scrap door lock actuator rod sharpened at the end for my probe. I use two different colored 12 volt LEDs to distinguish positive from negative when probing. Used scrap 12volt outlet socket to plug in to car 12volt/cigarette lighter like a power probe. I even threw in a rocker switch to throw out +and- signals..Gary Sather

Thanks for the thanks rasa. Like you, I have (always used) a DMM or older analog multimeter. I used to think test lights just provided one "cheap" MM function. It wasn't until I got on here that I eventually realised their advantage thanks to Howard's (Howie II's) writings. I realised what I wrote above about a LED test light except a MM/DMM has infinite impedance (well, at least tens of megaOhms). I had always realised that a MM could give a deceptive reading eg, is that 12.6V reading through a relay coil or bulb? But I hadn't (permanently) realised that a test light could solve that. IE - we usually want to know if it's a power source (a low impedance 12V or GND) or if that path includes some component (bulb, relay coil etc). A DMM has no chance of detecting that except by removing power from the test circuit and measuring resistance, and then matching that resistance to what is there. Or perhaps connecting a resistor and measuring its voltage drop. Now IMO a test bulb is much quicker and simpler. Sure, the bulb needs to be of similar resistance to the path under test, but coincidentally the most convenient & common bulb (2-3W) of ~~50R is similar to a lot of open circuit source resistances - ie, to a floating 7W or maybe 21W bulb, or a 60R to 400R relay coil etc. A 10:1 ratio should be easy to detect, hence a path range of ~4R to 400R. And though a 10% dim or less bright bulb may be hard to detect, simply change the power-side of the test bulb to the opposite supply (+12V else GND). [ I am assuming that ~1V will light the bulb enough to detect, but you get the point - it may require a bit more. ] With different line impedances a ~3W bulb may not be suitable. That's where a LED version might be handy, though for vehicle electrics (not electronics), usually it's a case of using a headlight instead (eg, starter solenoids). But aside from all the above, for most testing, IMO a bulb is the quickest way to test for power or GND. You don't even have to look directly at the bulb to see if it lights or not. Have you ever used a DMM only to slip off the target when you try to read the display, then try to get the probe back on that small target without touching other wires etc? With the light, you just concentrate on the probe touching the target. That covers the two advantages with a test light - namely the ease of "reading", and the "non-solid GND or +12V" aspect. (And it was the latter that I realised thanks to the12volt (ie, Howard). The 3rd advantage is its simplicity. If stuck somewhere without a DMM, it should be relatively easy to make a test light - borrow a dash or clearance bulb etc.     BTW - I drive a 48 year old vehicle. Modern cars with LEDs are somewhat different - both to borrow & remove, and their higher resistance. But the latter probably doesn't matter for power continuity testing in emergency situations. And I'm talking about a "raw" test light, not those with other functions or properties - like my "remote sensing" test light for AC voltages (that I only bought last year because of temporary employment servicing and repairing white goods). Sorry for all my blah blah verbiage, but this was another but rare "recent" learning. I realised that those cheap, bullsh & crappy test lights were not bullsh nor crap. In fact in many ways they are far superior to MMs for vehicle electrics testing. In retrospect, MMs really are NOT suited to typical vehicle testing when compared to test lights. (Maybe FIRST test with a test light. After that maybe a DMM to test resistances or voltage drops...) Also BTW - O forgot to add earlier that "R" meant Ohms (eg, 470R, 1R2 etc), but I guess you knew else figured that.    An amusing closing?... I still have not built nor obtained a 12V (DC) test light despite my high priority intentions to get one after joining the12volt. But since I still haven't completed my 1988 distributorless CPU ignition, I figure a few years ain't too bad. (LOL!) PS - my '88 ignition was almost resurrected in 2005 when I thought I'd have to demonstrate to a certain (qualified!) audience that a self-tuning ignition and EFI was indeed possible, as was sequential firing with a mere SINGLE crank Reference mark (ie, no CAS). But then I discovered limited self-tuning had been in production since at least 1990. And because some laughed at the suggestion of the latter CAS-less sequential, when I found another that knew, we decided to keep quiet. But I also think we enjoyed laughing at those that laughed insisting it couldn't be done. (Let those that don't know continue in ignorance. That is NOT my usual philosophy, but CASless sequential and the Greenhouse Effect (climate change?) are my two exceptions.) LOL - My PS for that incidental or hijack closing is longer than the rest of my reply!

I don't see any reason why anyone would say EFI without a cam signal would be impossible. As long as you have a way to denote top dead center you don't need a CAS (cam angle sensor) for sequential or batch firing. However, you should still have one. It's a failsafe. In a CASless EFI system if you lose your crank sensor you are done - the motor won't run. With a CAS your ECU will resort to the CAS signal and (most likely) batch firing to get you somewhere to diagnose / fix the problem! Cam angle sensor - not needed but sure nice to have! Sorry for going completely off topic on that one! Kevin Pierson

I have to admit I actually used my DMM yesterday but I was checking something on a tracker unit using low current and I didn't want to draw too much juice via the bulb. The answer to the OP's question is a MAC 120, with POS and NEG clips and a probe. I have a pair of (red and green) 12Volt LED bulbs via diodes gives me the same as KP's post, except they are for home use. I still say that provided you know what you're doing you can't beat an (incandescent) bulb test light for exactly the reasons Oldspark said.

And there you have a reply from my test-light Master. Can't beat that! (But hang on, he actually used a DMM... LOL!) And to think Howard reckons I covered it... (though I'm sure Howard (and others) would convey the lot in a succinct few-line summary). And sorry for a return to this next hijack, though it may be of interest to electronics or IT students & enthusiasts... KP - the CASless issue was restricted to sequential injection and ignition. Although a CAS isn't required for injection (that can be anytime during the cycle as often done when cranking, though injection when NOT on the intake stroke may be wasteful), it is traditionally used or required for sequential firing (injectors and ignition). Those jocular dudes reckoned you need a CAS to determine which cycle you are on - eg, TDC ignition or TDC dead (post exhaust). But some know how to determine that without a CAS. [ And I'm not talking of mark-less ionic sensing systems - just plain every day ignitions with carbies or EFI; and thinking about it now, electronic diesel injection systems (though I wouldn't bother with sequential ignition for them). ] In fact the method can be an independent add-on (except for IgCoil addition or injector intercepts, and batch or wasted fire injection volumes). I was thinking of building a unit after I ditched a distributor conversion to CAS (only because of "theoretical" reservations about the temp spec of the added Hall Effect sensor), but decided that the full programmable ignition timing to replace mechanical timing was my higher priority (and that is on a needs basis, but having since sourced alternate electronic distributors with spark that rivaled my (cheap sequential) CDI ignition....) But thanks to the laughter, that has been relegated back to its 1988 status - ie, pending or rount-tuit. The only difference now is my proposed PIC 08M2 instead of a 68HC11... I mean 68HC12... FYI - the other evolution is the sensing. An edge-filed ring gear tooth replaces the need to fit a magnet or some second sensor as the Reference signal.   And the gear-tooth sensor(s) can be fitted to the starter motor, hence merely a starter remove & refit instead of separating solid bell housings etc. The 2nd sensor is optional and merely a redundancy feature which can also provide a warning that either sensor is failing. PS To clarify: Not knowing or understanding is totally cool. It's the arrogant ridicule, ignorance or insistence that I object to. Like the shop owner that last year(?) kept insisting fluorescent tubes could not be dimmed (because of their stater & ballast nature) DESPITE me telling them I did it back in 1974 - and offering to explain how - or people that provide garbage statements based on some source's statement or definition yet do not answer resultant queries... I now let those continue in ignorance and enjoy their illusion until they catch up with reality. (Ironically they often don't. You'd think self-professed masters would notice...) Meanwhile others get good belly laughs or progress the said issues. I wonder if that's less tolerance with age, or a better balance of effort and reward. I think it's the latter.