Flashing LED (555) when signal goes low - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: December 22, 2022, 02:25:14 am
content: I want to flash a LED when a 12V switch is turned off (like a theft alarm when you switch off the ignition). I'm powering the 555 from a permanent 12V. The switched 12V is either grounded or floating (have to measure, but the wires are in a tight space).I have drawn up the standard 555 flashing LED example, and was thinking of using a voltage divider (12V->5V), feed that into an inverter and feed that to the 555 reset pin, but the 12V might fluctuates quite a lot. And I don't think it's a good solution.Any elegant way of doing this without inverter? Perhaps with the control pin?

author: Ian.M
author_rank: Super Contributor
timestamp: December 22, 2022, 02:56:17 am
content: Yes, as long as the permanent 12V never spikes above the 555's max permitted supply voltage or below 2/3 of its normal voltage. LTspice sim attached.

author: jonovid
author_rank: Super Contributor
timestamp: December 22, 2022, 02:57:56 am
content: for a 555 use pin 4 and a transistor to invert the standby power.other chips may have a better outcome for this type of application.other can help you on this one.

author: Ian.M
author_rank: Super Contributor
timestamp: December 22, 2022, 03:18:46 am
content: Plan B! Get a flashing LED and simply connect it between the unswitched and the switched 12V lines. It will put a small current into the switched 12V line when the switch is off, but if there is normally more than a few hundred mA load on the switched 12V it wont matter.

author: okw
author_rank: Frequent Contributor
timestamp: December 22, 2022, 03:44:57 pm
content: Simple and good, but I don't wanna mess too much with the ignition signal (it's automotive). Can't risk anything happening with the ECU.Also, the flashing LED's I have seen flashes rather fast. I want the steady, slow "alarm engaged" type pulsing.Would a mosfet shorting the 555 reset to ground be a good solution (ignition signal to base)?

author: okw
author_rank: Frequent Contributor
timestamp: December 22, 2022, 04:14:38 pm
content: Would this be the correct?

author: ledtester
author_rank: Super Contributor
timestamp: December 22, 2022, 04:57:36 pm
content: You're going to want a resistor on the base of the Q9 to limit current.Or use an N-channel MOSFET.I'm not sure how much current the 555 uses while held in reset mode, but you might consider using the CMOS version. Quiescent current is much lower -- around 250uA vs. 5-10mA for the standard 555.Finally, for the ultimate in power reduction I'd look for a way for the IGN+ signal to completely switch off the power to the 555. For example, in this design the 555 is powered off until an input signal comes in:https://www.ednasia.com/555-timer-design-consumes-zero-current-in-quiescent-mode/But it's for monostable mode, not astable mode.

author: okw
author_rank: Frequent Contributor
timestamp: December 22, 2022, 11:26:02 pm
content: Maybe this? Cuts off power when IGN is high.Here is also a much cheaper based on optocoupler. Would this work?

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2022, 04:21:57 pm
content: How about the 4011? It should use much less power than the power hungry 555.The 4001 would be better because the input becomes inverting, so the spare gate can used in parallel to give a lower output impedance for the LED.

author: okw
author_rank: Frequent Contributor
timestamp: December 30, 2022, 03:27:53 pm
content: The 4001 seems promisingThanks for the help.Is it good practice to put a 1k or 10k on disable signal? Like you limit the current on transistor base.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2022, 06:20:06 pm
content: Quote from: okw on December 30, 2022, 03:27:53 pmCMOS gates have a very high input impedance. The only resistor which might be required is a pull-up/down, to ensure it never floats, in which case it'll pickup noise.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 02, 2023, 09:44:42 am
content: You really do need a series resistor (say 47k) beween the gate input and the external auto electrics Surges can be very high. Read up on "Alternator Load Dump".For the same reason you need local over voltage protection for the 4011 Vdd pin. eg a low leakage TVSA small cap (say 10n) to ground from the gate input to ground is also advisable to prevent false triggering from system noise.

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2023, 10:11:19 am
content: Its a CMOS IC - so its own power consumption is pretty low. A suitable power supply for it could be as simple as a 1W 8.2V Zener, cathode positive across its power pins, fed by a dropper resistor from the vehicle 12V. Lets assume a 3mA drain is acceptable (and use a biggish reservoir cap to allow the LED to be pulsed with a higher current). A 2W wirewound 1K6 resistor feeding it will definately survive a worst-case 12V load dump and a 1W one *probably* will assuming the usual wirewound resistor 5x short time overload rating. All other supply transients will be effectively filtered.

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2023, 11:15:51 am
content: There is some debate about how much of an issue load dump is in modern cars.I would use a zener diode with a slightly higher breakdown than the nominal voltage, say 15V, a 1k resistor and a 10µF capacitor, which would keep the standby current low.

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2023, 11:32:55 am
content: Modern cars almost invariably have central load dump protection, e.g. by the use of avalanche diodes in the alternator. Good luck if you get the alternator rebuilt by a hole-in-the-wall auto electrics workshop, who will happily press anything that fits into the diode plate!Your low standby current approach is probably better if that 1K resistor can withstand the clamped load dump pulse. Even if the vehicle is reasonably maintained, such pulses can be fairly common if the battery is aging, or cold and near flat (e.g. after a winter jump start), whenever a heavy electrical load switches off. e.g. headlamps or electric radiator fan, due to the higher than normal battery impedance.

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2023, 10:41:15 pm
content: For a 12V system, a 1k resistor and a 15V zener, the peak power dissipated in the resistor is only 5.2W, and under 400ms, which is less than 1J, just doing a rough calculation. It should be easy to find a small resistor which won't blow up.https://itecnotes.com/electrical/electronic-simulating-a-load-dump-circuit/

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2023, 11:11:02 pm
content: Yep. Most wirewound resistors are good for a 5x overload for one second. Pulse rated ones can typically withstand a 10x overload for a similar timespan. Thick film resistors can also be capable of good overload performance, but check the specific datasheet.