Amplify/invert pulse for 4047 retriggerable monostable?

[lentilbake]

I have a hopefully simple problem that I'm struggling to solve, I'm pretty rusty at electronics and don't really have the time to re-learn everything I used to know!

Anyway, I have a negative going pulse from a Geiger Counter of about 0.5V and 50 micro-second duration(see attached pic) which I want to 'stretch' by triggering a 4047 chip configured as a retriggerable monostable. I want to use the output of the 4047 to drive a piezo beeper or some such so that each pulse gives a beep. I can't seem to trigger the 4047 with the pulse and I assume it's not quite the right shape being negative going and only 0.5V or so? What I need (I think) is something to invert and possibly amplify the pulse, a transistor inverter perhaps? I'm running the 4047 circuit on a 9V battery and this triggers/retriggers fine when I manually pulse the input so I think that circuit is working fine it's just getting it to trigger using the pulse from the counter.

Any help would be very much appreciated.

Thanks

[pebe]

Your problem is that you have a pulse that goes from 0V to -0.5V, whereas the threshold for a 4047 input to change over is somewhere between 45% and 55% of Vdd. You can overcome the problem by fitting a 100k pot across the supply and connecting its wiper to the 4047 input. Then starting with the wiper at Vdd, slowly reduce the voltage at the slider until the input triggers. Then back off slightly until it almost triggers. Now connect a 1nF cap between the pulse source and the pot slider. The 500mv negative-going pulse will then trigger the 4047.

Best of luck.

[lentilbake]

Thanks very much for your help, seems to work fine and was very simple to understand and implement.

Cheers