Momentary LED Indicator upon switching, will this work like I think it will?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,853
Have an auto darkening welding hood. It comes with an internal CR2032 battery, which will eventually go flat from just sitting around. Happened twice. The first time it happened I forced the lens assembly open to gain access to the PCB where the original 2032 was attached. Finally got it disconnected and wired up a new 2032 in a holder to the board. It worked. The holder was then glued to the welding hood. As before, the battery went flat. When I pulled the battery out of the holder the holder came loose from the hood. Of course I put another 2032 in and it's working, but I've decided to prevent another battery from going dead I want to put a switch on the hood.

Here's what I'm thinking: A SPDT slide switch to turn the darkening lens on and off. When I switch it ON I would like an LED to come on momentarily then extinguish. Timeframe is not critical. Anywhere from half a second to five seconds (not firm on that long, but long enough to observe that it has switched ON. THEN when I switch it OFF I'd like a similar situation where a second, different color LED lights up momentarily, then extinguishes. This is the circuit I've come up with. Your critique is appreciated.

CR2450 Data Sheet Here:
1669050046545.png
 
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Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,853
OK, change in colors: Red remains Red, Green becomes Yellow. My Red Superbright LED's have a 1.95Vf and Yellow has 2.01Vf. Green has 2.92 and may struggle to light in lower voltage situations. Haven't decided on capacitance or resistance yet. Just starting out on this project.

I rarely weld, which is the reason for the switch. Welding without the hood functioning can be hazardous to the human eye, so the indicators to indicate when I've switched it ON or OFF. That way I don't blind myself or run a battery down needlessly. I do have some welding to be done in the next week or so. Have ordered a CR2450 holder and battery. I'll find a switch hanging around from some of the many things I've pulled apart. Gosh I gotta de-junk my workbench.
 

crutschow

Joined Mar 14, 2008
34,285
If you have room, I have used CR123 cylindrical lithium batteries (in a holder) to replace lithium coil cells.
They should have about three times the life of a CR2450.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,853
Thanks crutschow. With a switch a 2450 should last years, given that I don't do a whole lot of welding. What I'm more interested in learning is if the circuit posted will do what I'm hoping it will.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,853
Just found some old CRV3 batteries. made them fit in a common 2X AA holder and tested with just a green LED, no resistors. The LED was bright but not brilliantly bright. Red has a good brightness without a resistor as well. Going to find some caps and test for times. Without resistance I wouldn't know how to calculate charge times for the cap.
 

MrSalts

Joined Apr 2, 2020
2,767
I'm having trouble seeing how you will ever get 2.xV across the green LED to light it up. And, if it is lit up, the current limit would not be the relatively high internal resistance of the button cell, but the much lower ESR of the capacitor.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,853
Tinkering with the circuit. As shown, when the switch is energized a 200µF (10V) electrolytic cap charges and the green LED briefly flashes. As said before, I don't care how long it's on, a brief flash is fine. But I'm having trouble getting the red LED to flash when the switch is back to the off position. OH, and I removed the resistors all together. Ignore the resistors in the schematic.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,853
Though wired up as shown (less resistors) I can get a green flash but not a red flash. For the diode I've used a 1N5818 Schottky diode. No going. Not giving up - but I really need to get out to the shop and continue building my lumber elevator. The shop is on the second floor and I hate carrying 3/4" plywood sheets up the stairs in the cold.

the current limit would not be the relatively high internal resistance of the button cell, but the much lower ESR of the capacitor.
May be the cap I used. Will bang around to find something NOT electrolytic. The drawing shows a non-electrolytic. My tests have been with such an animal.

See you all tonight.
 
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