A 555 wired as a re-triggered one-shot, where the timing period is longer than the turn-signal flasher period?I think that circuit will need a amplifier. The voltage on C6 will fall slowly, and there will be a large range of voltages where Q7 and Q8 are both on.
The idea is correct.
I think that circuit will need a amplifier. The voltage on C6 will fall slowly, and there will be a large range of voltages where Q7 and Q8 are both on.
The idea is correct.
Good point. If so the output transistors need to be changed to P-MOSFETS.I think you will find that the amber and white LEDs are common cathode not common anode.
Yes, I need to add a diode in series with the base.The -5V VBE reverse voltage of the 2N3906 transistor will be exceeded I suspect.
If they do oscillate it would only be momentarily during the turn on or turn off transitions and it wouldn't be visible.There are no gate stoppers, so the MOSFETs may oscillate.
I'll add a 0.1μF ceramic.There is no supply line HF decoupling.
What threshold??The voltages on the pins of the 555 will exceeded the 555 substrate diode threshold at first turn-on.
I think resistor R3 and capacitor C3 will take care of that.The automobile 12V supply line is effectively connected directly to the 555 which means that hash etc on the 12V supply line is likely to blow the 555.
Good point.At first turn on there will be around 1.2A going through the 1N4148 diode which is unwise. A 1N400x rectifier diode would be much safer.
I've never seen that limit in the data sheet but I'll change it to the 10nF shown in the example circuit.It is unwise to decouple the 555 control pin with a capacitor larger than 10nF.
The circuit is mounted outside the engine compartment so likely the only extreme temperature beyond the 555's 0-70°C limit would be in cold weather, and I suspect it will still operate.Automobile grade components have not been specified so the circuit will not meet the environmental conditions for an automobile.
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Clever implementation.Below is the LTspice simulation of a somewhat simplified version of spec's circuit.
The time-constant R1C1 determines the delay until the running lights return after the turn signal stops. The delay needs to be slight longer than the ON time of the turn signal.
C2 prevents short transient current glitches that appeared in the DRL LED when the turn signal was blinking.
View attachment 100287
Thanks.Clever implementation.
spec
But the 555 circuit is more accurate.Thanks.
My goal when designing circuits is the quote attributed to Einstein: "Make things as simple as possible, but not simpler."
or KISS in the popular vernacular.
In this case I think your circuit approach fits that goal better than the 555 circuit.
You're throwing my own words back at me.But the 555 circuit is more accurate.
spec
What threshold??(substrate parasitic diode)
No, I can't find it on data sheets either, but at one time there was a limit of 10nF to limit the surge current at turn-on through the top resistor of the three internal voltage divider resistors. Maybe the resistors have been beefed up on modern 555s.I've never seen that limit in the data sheet. (10nF limit on control pin decoupling)
I had envisioned the unit being mounted near to the lights so that the existing signal wire simply plugged into the unit and two short wires connected to the light unit. An earth wire would also be required.The circuit is mounted outside the engine compartment so likely the only extreme temperature beyond the 555's 0-70°C limit would be in cold weather, and I suspect it will still operate.
This is true in essence, but taking the point to the extreme the 555 may go short circuit, under high temperature for example.Even if it failed to trigger, one or the other of the lights would still blink so there's no safety hazard involved.
Below is the circuit with the above modifications, including P-MOSFETs to drive grounded LEDs with a common cathode. Note that the MOSFETs should be standard type with a Vgs(th) of at least 2V.
Logic-level type MOSFETs may not fully turn off.
Incidentally, it may be necessary to put a resistor in parallel with each LED to make them take the same current as filament lamps if the automobile electronics uses current sensing to detect a blown lamp. Of course, this assumes that the original lamp was a filament type.
I fully understand that........................
One parameter that is missing from the absolute maximum ratings is the maximum/minimum voltage on any pin with reference to the supply lines, so unless otherwise stated, you can only assume that the allowable over and under voltage is zero volts.
Generally the pins of a chip are isolated from the substrate by parasitic diodes. These diodes are fairly delicate and not designed to conduct current so they are easily damaged.
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I don't think the circuit will exceed the 555 limit of 70°C (158°F) where it's located at the front of the car (unless it's parked facing the sun on a summer day in Death Valley).This is true in essence, but taking the point to the extreme the 555 may go short circuit, under high temperature for example.
I fully understand that.
I just didn't see where that would occur in the circuit I posted.
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