Need help with intermittent circuit failures - JK Flip Flops

Jason S · 2006-06-06T12:54:56+00:00

Hi,I designed this simple circuit where it controls a gearhead motor. The circuit is supposed to stop the motor for 1 sec (when triggered), and then upon reactivation, the motor is to change its polarity and spin the opposite direction, and so-forth.What the circuit should do:1) Motor spins (let's just say clockwise) until a timer is triggered.2) Timer cuts off power to motor breifly (enough time for it to stop spinning) for 1 sec.3) 1 sec lapses.4) Relay then changes state, changing motor polarity.5) At the same time, power is reapplied to the motor (should spin anti-clockwise this time).6) Cycle repeated until timer triggered again.The motor is to stop briefly before the polarity change-over for obvious reasons (needs time to stop spinning before changing direction!).The circuit consists of a 555 timer IC connected as a monostable which is triggered by a reed switch and magnet (when magnet is nearby, the reed switch closes and therefore activates the timer). The timer gives out a 1 sec 'high' pulse and then changes back to low state until it is triggered again by the magnet. The timer's output controls transistors that affect the behaviour of a JK Flip Flop (for direction control of the motor, via a DPDT relay), and power for on/off of the motor.Because I need the JK Flip Flop to change the relay polarity *after* the motor stops for that 1 sec, I had to somehow invert the output from the 555 so the JK would flip-flop at the right time. Instead of using a 4069 inverter IC to do this, I connected up a couple of transistors with pull-up resistors to achieve this.The circuit works fine most of the time (like 80%), but it's unreliable, and I don't know what I can do to make it work properly every time. Most of the time the relay changes state 1 sec after motor stops (which is correct), but other times it does other wierd things, like the relay would act as if it's connected directly to the 555's output pin. Also, llthough very rare, the JK Flip Flop would even...

Jun 06, 2006 42 Replies

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Jason S 20 years ago

=) Ok, fair enough (sorry, it's just that this wasn't explained to me). That delay was there before and now it isn't - which is why I got a little concerned. But yes it works ... and nothing is overheating, yay!

I've been playing around with it, bit of a hiccup found (God, please let this be the last one so I can leave John in peace! lol). And again, I am unsure if you intentionally made it do this, so forgive me if it's not actually a fault! **If neither of the reed switches are closed at Power On, the motor won't start (the LED is off too). But if one of the reed switches are closed and then power is applied to the circuit, it works fine. What's the go with that?

Jason.

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John Fields 20 years ago

Not a problem! :-)

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Jason S 20 years ago

Hi John, Tried that. No luck :-( I used a 0.1uF like you said, but a greencap... that ok? Tried both U1-2 and U1-6 (two separate caps) as well, no good. It seems to be very intermittent too... like 1 in maybe 10 shots the motor would start.

Jason

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