I am making a tester for automobile diagnostic ports. It checks for ground and 12V (easy, green LED and resistor for correct hookup, red if + and - are reversed). But also for 12V square waves which go to the tachometer (systems before around 2002 had those, now it is all coded packets). Those are going to be, in a car at idle, in the 33-67 Hz range. Any light would flash to fast, and I don't trust concluding that an LED across the tach to ground being on means it is being pulsed.
Aha, just divide these frequencies down quite a bit, so the light flashes about every second or half second or so. How about using 12V power. a 555 in monostable mode as a frequency divider. I have a handful in a bin along with Rs and Cs and LEDs of the sort which would be used. So I bread boarded a standard circuit: the square wave into pin 2 (I've played with a pull up resistor of 100K to +, and a pull down to ground, don't think I should need either). The output from pin 3 to an LED and a resistor to ground. Fiddling with values, 100K resistor from + to pins 7 and 6, with a lowish microF (at first, 2.2, as it was in the bag on my desk) from there to ground. Pin 5 with 0.01 to ground. Pins 8 and 4 to +. Pin 1 to ground. I had to read up to remember this stuff.
But it didn't flash. It would if I ran my square wave generator at 1 Hz - gives a nice 1 Hz flash (which is probably what I want anyway, rather than something shorter and challenging the persistence of vision and whatever linger time - maybe none? - in an LED).
But when I turned up the frequency up beyond 3 Hz (flashes faster at 2, and faster yet at 3), it gives the appearance of being on all the time.
Upping the capacitor to 22 microF has it flashing with 5 Hz input at about once per 3 seconds for five flashes, and then a pause while it starts in again
With no capacitor, it flashes at the input speed, becoming a blur at about 30 Hz.
Which has me wondering if this is happening: The falling edge of the input turns the LED on for a period defined by the R/C. But when it has ignored (if it has) some pulses, and is reset and ready to flash again when triggered, the trigger comes so close after the change of state that it appears continuous? Maybe a scope or frequency meter could measure this, but it is too fast for the eyeball?
I know I could use a CMOS divide by 12 chip, or a variety of other chips, flip flops and so on) to put faster pulses in one end, and get slower ones out the other. But the 8 pin DIP 555 is just the size I want for something perhaps housed in the back end of an OBDII connector. No SMDs, but small.
So can I get the 555 to do what I want? Or do I need to try something else. If F is between 25 and 100 Hz, turn on the flasher LED? This is stretching my cook bookery.
Walt Fricke
Aha, just divide these frequencies down quite a bit, so the light flashes about every second or half second or so. How about using 12V power. a 555 in monostable mode as a frequency divider. I have a handful in a bin along with Rs and Cs and LEDs of the sort which would be used. So I bread boarded a standard circuit: the square wave into pin 2 (I've played with a pull up resistor of 100K to +, and a pull down to ground, don't think I should need either). The output from pin 3 to an LED and a resistor to ground. Fiddling with values, 100K resistor from + to pins 7 and 6, with a lowish microF (at first, 2.2, as it was in the bag on my desk) from there to ground. Pin 5 with 0.01 to ground. Pins 8 and 4 to +. Pin 1 to ground. I had to read up to remember this stuff.
But it didn't flash. It would if I ran my square wave generator at 1 Hz - gives a nice 1 Hz flash (which is probably what I want anyway, rather than something shorter and challenging the persistence of vision and whatever linger time - maybe none? - in an LED).
But when I turned up the frequency up beyond 3 Hz (flashes faster at 2, and faster yet at 3), it gives the appearance of being on all the time.
Upping the capacitor to 22 microF has it flashing with 5 Hz input at about once per 3 seconds for five flashes, and then a pause while it starts in again
With no capacitor, it flashes at the input speed, becoming a blur at about 30 Hz.
Which has me wondering if this is happening: The falling edge of the input turns the LED on for a period defined by the R/C. But when it has ignored (if it has) some pulses, and is reset and ready to flash again when triggered, the trigger comes so close after the change of state that it appears continuous? Maybe a scope or frequency meter could measure this, but it is too fast for the eyeball?
I know I could use a CMOS divide by 12 chip, or a variety of other chips, flip flops and so on) to put faster pulses in one end, and get slower ones out the other. But the 8 pin DIP 555 is just the size I want for something perhaps housed in the back end of an OBDII connector. No SMDs, but small.
So can I get the 555 to do what I want? Or do I need to try something else. If F is between 25 and 100 Hz, turn on the flasher LED? This is stretching my cook bookery.
Walt Fricke