# Generate single pulse on active high signal

#### JimWright

Joined Aug 14, 2011
8
Hello all, I've been working on a circuit for a while now, and so far my efforts have fallen flat. Here's the background on what I'm working on:

I have a digital video camera that I need to set up to start recording soon after it receives power. The camera does not turn on automatically when power is applied, the power button (contact switch) needs to be pressed. Another contact switch must be pressed to start recording.

What I've designed so far uses a 74HC4066 switch to trigger the camera buttons, that part is simple enough. My problem lies in triggering the 74HC4066 inputs reliably.

So far, I've set up two different monostable multivibrator circuits, my problem with each is that the input for each needs to be in the form of a pulse, and designing something elegant that works has eluded me.

I wouldn't have thought this would be a difficult circuit, my core issue seems to be that my trigger input is going high and staying high. So, rephrasing what I'm needing, I'm looking for a circuit that can use an SPST switch (non-momentary) to generate a single pulse of a desired width.

Thanks!

#### praondevou

Joined Jul 9, 2011
2,942
After you turn on power, what's the sequence/timing?

1. After power on, after what time you activate the camera power button?
2. How long you want it to be activated?
3. Then, after the camera power button has been released, after what time you want to activate the record button?
4. For how long shall it be active?

5. I assume the camera buttons are momentary switches, is that correct?

#### JimWright

Joined Aug 14, 2011
8
Hi, praondevou. What I'd like is a 5 second delay after power on to generate the pulse that turns the camera on, and another delay of 12-15 seconds from power on (Or 7-10 seconds from the first pulse) for the pulse that activates the record button.

Pulse width should be between .5 and 1 second in duration, camera buttons are momentary contact.

#### Pencil

Joined Dec 8, 2009
272
I wouldn't have thought this would be a difficult circuit, my core issue seems to be that my trigger input is going high and staying high. So, rephrasing what I'm needing, I'm looking for a circuit that can use an SPST switch (non-momentary) to generate a single pulse of a desired width.
Look for an "edge trigger" to generate a short pulse from your SPST switch
to trigger a monostable.

Attached is a circuit that may be of some reference. The purpose of
this circuit was to generate two pulses of different lengths starting from
the same input event. In this case a relay was used to start the timing
cycle.

Operation was as follows:
1. RELAY (K1) OFF
OUTPUT 1 LOW.
OUTPUT 2 LOW.

2. RELAY (K1) ON
OUTPUT 1 LOW.
OUTPUT 2 LOW.

3. RELAY (K1) OFF
OUTPUT 1 goes high for x seconds then returns low.
OUTPUT 2 goes high for x seconds then returns low.

I realize this is not exactly what you need, but you may get some
ideas from it.

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#### praondevou

Joined Jul 9, 2011
2,942
If you really want the timings you mentioned, 5 sec, 1 sec, 8sec, 1 sec, you need 4 timers. I drew a schematic that has 4 x 555s. You could also use 2 x 556s.

The two timers on the left are triggered when the circuit is powered on. After their time delay they trigger the other two 555s, one after 5.2 seconds, the other after 13 seconds.
The outputs of the two 555s to the right go HIGH for 1 second. You need to interface them to the switches.

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#### JimWright

Joined Aug 14, 2011
8
The edge trigger was the part that I was missing, I'll play with that tonight when I get home and see if I can adapt that to the circuit I already have.

Thank you both for the suggestions!

#### JimWright

Joined Aug 14, 2011
8
Hey all. I've done some testing and managed to get a working prototype. So far I've only built half the circuit I need, but I have working a 4.95 second delay from power up, generating a pulse of about 1.3 seconds. Assuming I've got the math worked out right, but visually checking the output hooked to an LED this seems close.

I ran into some initial problems due to poor math doing the RC time calculations, once I figured my decimals were in the wrong places, the numbers finally made sense.

I've done a rough schematic, my drawing skills are about one step above a napkin drawing (sorry!), but this should get across what I'm doing.

I've incorporated the edge trigger, which seems to be working fine. I'm using a 4066 quad analog switch with two RC timers to get the initial delays of 4.95 and 9.9 seconds, the output from this is a negative pulse, both of which run to an 74HC123 monostable multivibrator, which uses an RC circuit to output the pair of 1.3 second pulses, one at 4.95 seconds from power up, the other at 9.9 seconds from power up.

These pulses both go back to the 4066, to trigger the two momentary switches that will attach to the digital camera to activate the power and record functions.

On the 74HC123, the RC circuit on pins 3 and 11 keep the chip from outputting a pulse at power on.

I'll probably play with the pulse delays and durations this weekend when I finish the prototype circuit and get it wired to the camera, if things change significantly I'll revise the schematic and upload again.

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#### JimWright

Joined Aug 14, 2011
8
Disregard the above schematic, I spotted an error on the 74HC123, pins 6 & 7, the RC circuit was drawn wrong. It's corrected in this version.

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#### Pencil

Joined Dec 8, 2009
272
You should add some .1uF "bypass capacitors" between
VCC and Ground, positioned as physically close to the VCC pin
on each I.C. on the board in the final design. Look here LINK. This
is a sticky on the top of the General Electronics Chat page.

and designing something elegant that works has eluded me.
I don't think it is eluding you any longer.

Last edited:

#### Pencil

Joined Dec 8, 2009
272
I just noticed you you left out the
diodes that are parallel to the resistor
on the edge trigger portion of you circuit.

These are necessary as a "voltage spike" is
generated when capacitor discharges/charges.
See the example I posted above, it is labeled D1.

In simulation this spike is 2x Vcc, which could possibly
damage the I.C. that is being triggered.

#### JimWright

Joined Aug 14, 2011
8
You should add some .1uF "bypass capacitors" between VCC and Ground, positioned as physically close to the VCC pin on each I.C. on the board in the final design.
Excellent suggestion, when I lay out the final board I had planned to add these. The power supply is actually only going to be about 2" of wire away, I'm using an LM2596 power module to drop down a 12v power source, this will also be powering the camera. My circuit should have a very low current draw, but the bypass caps are always a good idea.

I just noticed you you left out the diodes that are parallel to the resistor on the edge trigger portion of you circuit.

These are necessary as a "voltage spike" is generated when capacitor discharges/charges. See the example I posted above, it is labeled D1.

In simulation this spike is 2x Vcc, which could possibly damage the I.C. that is being triggered.
I saw this on your drawing, but other schematics I referred to didn't show this for the edge trigger, I figured there was a good reason to have it there but the exact purpose wasn't obvious at the time. Also, I didn't have any diodes on hand when I was building the prototype. I'll make sure to include these in the next build.