Design help needed! paid! :)

Community PCB Design
1

Hi all,

What I thought was a simple project has turned into a headache…

Would anyone be interested in designing a simple debounced flip-flop that is always off on initial power on, and changes states with a momentary n/o switch?

Not sure what going rates are for design services, but happy to pay a fair rate.

The purpose is as follows… we have software video capture program, and we have an external usb switch mapped to start and stop recording via a simple momentary button switch. The rub is that there is no visual feedback that the button has been pushed, eg that recording is happening. Until the software coders get around to a send on start to the usb bus, we can’t get feedback out, so we have to fake it by having the same switch toggle an LED at the same time as it toggles the start/stop button in the software.

Having constructed several test circuits posted on various DIY electronics sites, I realize I know too little to make any needed changes to the designs…and we need it sooner than I am likely to learn all I need to know. Hence this post.

Some paramaters:

On power up of the computer, the LED should stay off, and on pushing the momentary button the LED should go on, and go off when the switch is pushed again. Very simple in theory. Not so easy in practice!

I used this bistable flip-flop http://ourworld.compuserve.com/homepage … /page9.htm and substituted a resistor for the left hand LED, but it is not reliably “left-handed” on startup. I also added the mentioned 1K resistor in parallel with the LED to make sure it really goes off as the flip-flop changes states.

So…the board should be either 5V or 12V, eg power supplied from a standard ATX power supply. It should have one set of terminals for the LED, and one for the switch. Otherwise choice of approach (555 IC, 4013 IC, a modified version of the one I tried) is up to you, as long as it does those few things.

Again, the crucial thing is that the LED is off when power is first applied to the system, as it needs to reflect the state of the “start/stop” button in software, which is always off at startup.

thanks!!

Charles

2

cda:
I used this bistable flip-flop http://ourworld.compuserve.com/homepage … /page9.htm and substituted a resistor for the left hand LED, but it is not reliably “left-handed” on startup. I also added the mentioned 1K resistor in parallel with the LED to make sure it really goes off as the flip-flop changes states.

Have you tried adding a small capacitor (1uf or so) from the base of one of the transistors to ground? That will provide a small delay on one leg so that the flip-flop always starts up in the proper state.

3

Thanks for the hint, I will try it now…I did read that somewhere but was unsure exactly where to put the capacitor.

There were other issues though…

12V from a battery worked fine…12V from an ATX power supply did not work, inexplicably as the voltmeter showed 12V ± .05V for both.

But then I am new to all this so maybe there is something about the converted current that is different than the battery current.

4

riden:
Have you tried adding a small capacitor (1uf or so) from the base of one of the transistors to ground? That will provide a small delay on one leg so that the flip-flop always starts up in the proper state.

Tried it, first on one and then on the other of the transistors. it’s still unreliable…70% off and 30% on at start.

Think what I need is someone who really understands what they are doing :slight_smile: I’m just humming a few bars here, none too successfully.

5

You might have to adjust the resistor values to account for the gain of the transistors and the applied voltage. If you still are having problems and have a 555 laying around, I’d try that circuit. The chip can source/sink up to 100ma. You will need to add a resistor and a capacitor on the reset pin (4) though.

6

riden:
You might have to adjust the resistor values to account for the gain of the transistors and the applied voltage. If you still are having problems and have a 555 laying around, I’d try that circuit. The chip can source/sink up to 100ma. You will need to add a resistor and a capacitor on the reset pin (4) though.

I’d love to experiment more, but I’m running out of time. I’d also like to have a few bells and whistles that I doubt I can get into myself, such as an IR switch option. Some of the other switches on that web page I quoted earlier allow multiple switches in parallel. “CMOS Toggle Flip Flop Using Push Button” for instance.

7

Try this and see what happens. I didn’t do any calculations on the component values so you may have to adjust some.

cmostoggle.jpg800×422 22.5 KB
http://i2.photobucket.com/albums/y30/an … toggle.jpg