Screw Switch with LED Indicators - Page 1
Forum section: electronics / beginners

author: Electro Dodo
author_rank: Contributor
timestamp: July 18, 2025, 09:49:55 am
content: HelloI try to build a 2 stage screw switch in which 3 nuts are connected through a bolt.The first nut is the Vin (6.8-50V).When the screw contacts the second nut, the green indicator should light up.When the screw contacts the third nut, the green indicator should turn of, and the red one on.I thought about using a p-channel mosfet in series with the green LED but ran into the problem that the Gate to Source Voltage of the mosfet would reach up to 50V.The highest Gate to source voltage for mosfets seems to be 30V.It seems like such an easy problem, but I could not find a simple solution.The best I could come up with is to use Two LDOs to drop the Voltage one both nuts, but I try to keep circuit as simple as possible, so this solution does not seem great.So, is there a better solution, or even an IC for it?

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2025, 11:20:03 am
content: Try this. R1 and R2 need to be 1W resistors due to the dissipation at max. Vin. R3 can be 1/4W or even less. The transistor can be any 'jellybean' NPN good for >= 200mA Ic. Its voltage rating doesn't matter as it never 'sees' more than D2's Vf. The resistors are chosen to limit LED If to about 24mA at max. Vin but still give about 1.5mA at min. Vin - enough if you use high efficiency LEDs. Any circuit to hold the current steady despite Vin changing would be a *LOT* more complex and less rugged.N.B. Vin must not be negative or AC.

author: nali
author_rank: Frequent Contributor
timestamp: July 18, 2025, 12:19:40 pm
content: Red LEDs have a lower Vf than green so something stupidly simple like this might suffice. When the screw hits the 3rd contact the LEDs are in parallel so the red should override the green (maybe put a diode in series with the green if the forward drops are too close)

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2025, 01:09:31 pm
content: That reverse biases the RED LED by nearly 50V when contact 2 (only) is closed, as the load is pulling down contact 1. Most LEDs are only rated for 5V reverse bias, although they typically survive quite a bit more.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 18, 2025, 02:16:01 pm
content: Could OP not just add a zener diode between source and gate (say, 15-20V)? Probably also reduce the value of the 10k resistor (e.g. 4.7k) so the zener regulates well by passing a little more current.

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2025, 05:47:05 pm
content: The Zener doesn't *need* to regulate well, it just needs to clamp Vgs at less than Vgs_max. For MOSFETs with +/-20V Vgs_max, a 12V or 15V Zener should do nicely, and is certain to give >10V gate drive. However, for Vin higher than the Zener voltage it introduces a sneak path from the second contact to the third contact (Vout), with no current limiting, so to avoid blowing the Zener you also need a small signal diode e.g. 1N4148 between the third contact and the MOSFET gate.

author: Benta
author_rank: Super Contributor
timestamp: July 19, 2025, 09:12:14 pm
content: I'd just do this (R1 represents your current sink):[ Specified attachment is not available ]

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 19, 2025, 10:07:10 pm
content: Quote from: Ian.M on July 19, 2025, 05:47:05 pmAh, I didn't think of that! With Vin>Vz, Voutwould end up being supplied with whatever Vin-Vzis. Yes, a diode would solve that problem.That would make the amended circuit: