Adding a digital timer... - Page 1
Forum section: electronics / projects

author: ximox
author_rank: Contributor
timestamp: January 20, 2012, 08:20:43 pm
content: Hi All,I built a little etching device. It's a simple power supply with a voltage selector (12/24) and an AC/DC selector. A stencil is applied to the part to etch, clipped to the positive end of the supply. A probe with a felt tip is attached to the negative side, dropped in electrolyte and used to etch the piece.This eats away the metal not protected by the stencil and you get a nice etch (for instance the makers mark on knives is done this way etc.). The proper technique is to press the probe on the metal for a few seconds (varies depending on the material), lifting it up to degas and repeating a few times. To obtain consistent results you should keep the application times constant.I would like to add to the device one of those digital timers (they usually have a start, stop, reset controls), the way I'd like it to work is for the timer to start counting as soon as the probe contacts the workpiece. When the probe is lifted the timer stops and after a second or so resets to zero until the next cycle.I am completely at a loss, especially considering that the probe can have either an AC or DC voltage running through it at any given time but the timer should work in the same way regardless. I'd like a solution without having to throw a microcontroller at it...Even if I use a microcontroller I can't think of a way to trigger the on/off/reset. I guess since I am shorting the leads, there must be a safe way of detecting it.Someone on a forum suggested to use a relay, not sure how I'd hook it up though and I'd still need to reset the timer.Thanks!Antonio

author: Mint.
author_rank: Frequent Contributor
timestamp: January 20, 2012, 08:49:04 pm
content: Bump. Interested

author: Rufus
author_rank: Super Contributor
timestamp: January 20, 2012, 09:21:06 pm
content: Quote from: ximox on January 20, 2012, 08:20:43 pmYou have not been very clear about your requirement.Are you trying to display the time the probe was applied?Are you trying to display the time 'live' during the application?Does the time need to stop counting if you remove the probe?Do you really need the time to reset after a second or so or could it reset when you next apply the probe which is probably simpler?

author: ximox
author_rank: Contributor
timestamp: January 20, 2012, 11:38:54 pm
content: All I really need is the counter to start from zero as soon as the probe is applied.Time does not need to stop when probe is removed, it's ok if it does though. Resetting when the probe is reapplied is fine.It's just more convenient than counting or looking at a clock. It's not a super precise thing but keeping the time consistent makes better etches.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 11:53:31 pm
content: Functionally, this seems easy. When the etch probe is applied current flows; when the probe is not in contact current stops.So you could have a sensitive current detector on the etch circuit that drives an electronic switch that starts and stops the timer. It would be easiest to reset the timer manually to zero when required, but an automatic reset could be contrived if necessary. The automatic reset might be triggered after the timer has stopped counting for more than a certain number of seconds, say.

author: ron
author_rank: Regular Contributor
timestamp: January 21, 2012, 12:06:33 am
content: You could use a darkroom timer and a foot switch. Time-O-Lite made some with a foot switch socket They time up to 60 sec and auto reset. If you insist on digital, use your toe.