Hello,
I'm new here, and have signed up as I am planning to build a timer circuit. It's a monostable, based on a 555 chip.
I want to produce an output that is high (+5v, with +5v being the circuit voltage) for around 4 seconds when an input voltage is applied. The input voltage will be +5v as well, but the input voltage will remain high for a long period of time (minutes or hours), and I need the output voltage to only go high for 4 seconds before going low again.
With apologies for the hand-drawn diagram, I think this is the circuit I need to build. It seems that the time of the circuit is 1.1*C1*R1, so 4MOhms and 1MicroFarad will give me a little over four seconds, which is what I need.
However I have two questions;
1. Will this circuit work given that the input pulse is much, much longer than the output pulse?
2. How do I make this triggered by a rising input pulse? It seems that 555's work on falling-edge pulses, which I can't give it. This would be terminal 2 on the timer (with a ? on the diagram).
(Background, should it be relevant; I have bought a bluetooth audio receiver to use in the car. It works well, but needs to be turned on by holding a momentary switch in for about four seconds whenever it is plugged in - just plugging it in won't turn it on. The momentary switch connects a terminal in the receiver to live.
I could mount the thing in the car such that I press the button whenever I want to use it, but that's inelegant and I'd like to make it happen automatically.
I'll run the thing off an ignition live via a USB supply (+5v), so I want the 'turn on' button to be pressed for four seconds whenever the ignition live (i.e. supply) goes live. The supply will clearly stay live for some period of time, but I don't want the output to stay live for more than 4 seconds as that will make the device reset.)
Thanks,
Oli.
I'm new here, and have signed up as I am planning to build a timer circuit. It's a monostable, based on a 555 chip.
I want to produce an output that is high (+5v, with +5v being the circuit voltage) for around 4 seconds when an input voltage is applied. The input voltage will be +5v as well, but the input voltage will remain high for a long period of time (minutes or hours), and I need the output voltage to only go high for 4 seconds before going low again.
With apologies for the hand-drawn diagram, I think this is the circuit I need to build. It seems that the time of the circuit is 1.1*C1*R1, so 4MOhms and 1MicroFarad will give me a little over four seconds, which is what I need.
However I have two questions;
1. Will this circuit work given that the input pulse is much, much longer than the output pulse?
2. How do I make this triggered by a rising input pulse? It seems that 555's work on falling-edge pulses, which I can't give it. This would be terminal 2 on the timer (with a ? on the diagram).
(Background, should it be relevant; I have bought a bluetooth audio receiver to use in the car. It works well, but needs to be turned on by holding a momentary switch in for about four seconds whenever it is plugged in - just plugging it in won't turn it on. The momentary switch connects a terminal in the receiver to live.
I could mount the thing in the car such that I press the button whenever I want to use it, but that's inelegant and I'd like to make it happen automatically.
I'll run the thing off an ignition live via a USB supply (+5v), so I want the 'turn on' button to be pressed for four seconds whenever the ignition live (i.e. supply) goes live. The supply will clearly stay live for some period of time, but I don't want the output to stay live for more than 4 seconds as that will make the device reset.)
Thanks,
Oli.