cannot reliably toggle flipflop when driven by phototransistor circuit

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Hi, I am trying to toggle a +'ve edge triggered JK flipflop (74hc109) ,
it toggles fine when the clock input is driven by a 555timer but when I
try to drive the clock input from a phototransistor (which is being
pulsed by the 555 driving an emitter) then most of the time it will not
toggle, just stays at a high level. But if the emmiter and
phototransistor are placed directly in front of each other - then no
problem, even though the switching voltage levels at the clock are the
same no matter what the distance between them (good alignment is
mantained). I have tried buffering the phototransistor output to the
clock and this improves the situation a little (more so when I connect
the DVM to monitor the clock voltage!!), but even still it is not
consistent. Does anybody have an explaination for this? I would greatly
appreciate any assistance

Brian C.


Re: cannot reliably toggle flipflop when driven by phototransistor circuit



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Might be a bit overkill, but you could use the opto to trigger a 2nd 555
wired as a monostable to drive the JK CLK



Re: cannot reliably toggle flipflop when driven by phototransistor circuit


Stumbled onto the fix soon after writing the above, I put a 22k
resistor in series between the buffer output and the flipflop clock
input and this solved the problem.


Re: cannot reliably toggle flipflop when driven by phototransistor circuit


But, still misses 1 out of every 50 pulses. Any gurus out there with
experience of this?


Re: cannot reliably toggle flipflop when driven by phototransistor circuit



Finally, for the sake of somebody else with a similar circuit, the
buffer output is placed as close to the flip flop clk input as possible
then tie the buffer (invertor in this case) output to ground thru a
0.1uf cap(multilayer ceramic chip) and then run a 5.1k resistor to
connect it to the clk input, works great.


Re: cannot reliably toggle flipflop when driven by phototransistor circuit



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Look at the signal with a CRO - it is the only way to see what is
happening - where are you ? if you are in the Northern suburbs of Sydney I
might be able to lend you a CRO

Regards
Richard Freeman



Re: cannot reliably toggle flipflop when driven by phototransistor circuit



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The Voltage levels must not be the same - if connecting a DVM improves
things slightly it sounds like the voltage is not going low enough.

what have you used to measure this signal ?
if you have not used a CRO then you cannot know what is really happening
here.

you may need to feed the signal through a schmitt trigger (my favourite is
the 74LS14 ) to ensure that it is toggling correctly - either way you need
to look at the signal with an Oscilloscope

Regards
Richard Freeman



Re: cannot reliably toggle flipflop when driven by phototransistor circuit


Thks for the offer of the oscilloscope, that is really the way to do
this, but I'm a long ways from Sidney unfortunately (virginia,usa).
Trying to do this project on the cheap if I can help it. I think most
of the issues I have are related to the use of high speed cmos chips on
a bradboard with all that stray capacitance and long turning connecting
wires. I've had some success so far using resistorcapacitor combos to
make it work, I suspect alot of these problems will reduce somewhat
when /if I move on to the pcb testing stage. Its been a long time since
I learned all this stuff. The voltage levels you talked about do swing
far above and below the thresholds in question but then that is when
the dvm is already connected, but a schmitt trigger may be something I
can try if I stop progressing altogether, will try to borrow a scope
from somewhere. Thks for your interest


Re: cannot reliably toggle flipflop when driven by phototransistor circuit



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Unfortunately, Virginia is a long way from Sydney too.


Andy Wood
snipped-for-privacy@trap.ozemail.com.au

Re: cannot reliably toggle flipflop when driven by phototransistor circuit


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If you ask nicely in sci.electronics.basics, you might get an old sub
20MHz scope for free, people toss this sort of stuff away quite
regularly, and will happily donate to a beginner.
Else you can pick one up for next to nothing on eBay.

Dave :)


Re: cannot reliably toggle flipflop when driven by phototransistor circuit


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Thks for the tip..


Re: cannot reliably toggle flipflop when driven by phototransistor circuit



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Good luck with your project but another thought occoured - it is usual when
using a Photo transistor to receive some sort of signal to run the signal
through a Bandpass (or at least high pass) Filter first - this should allow
you to add some gain after the photo-tranny and increase its selectivity.
The odd thing is though if you are placing a cap from the signal path to
Earth then this is effectively a low pass filter ....

Regards
Richard Freeman



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