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Review/Critique of Sequential Monostable Timing Circuit (555/556)

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rallyemax

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Hello!

After more than a decade away from soldering irons and electrons, I am trying to make a bit of a comeback. Not that I was ever anything more than a hack, but at least the math and physics came easy, but those are gone now.

I would like to get some input on a circuit I'm working on. It is an analog proof-of-concept for a project I intend to complete using an Arduino board (and eventually a standalone Atmel chip). The functional objective is to create what is called a "quick shifter" for a motorcycle -- a device that senses force input on the shifter lever and responds to the force input by cutting ignition to the motor for a precise but very short period of time (typically 50ms-75ms), which unloads the drivetrain while the throttle is still held wide open, which in turn selects a higher gear (the same force on the shifter that triggered the ignition cut preloads the transmission, and when the drivetrain is unloaded, the next gear is "sucked in"). It's a useful bit for racing, letting the rider skip blipping the throttle to accomplish the same thing (which has a cost in terms of concentration: "another thing" to have to do); it's also noticeably faster (everything else being equal coming out of a corner onto a straight, I've gained about 4-6 feet per shift versus a rider manually blipping the throttle, and after three shifts, a 15 foot advantage coming into a hard breaking corner is a wonderful thing!).

Electronically, the objective is to have the closing of a normally open switch break a 10A/12V circuit for precisely 60-75 milliseconds (either ~60 or ~75, to be determined after testing, and when the concept is exported to an Arduino, this will be adjustable), and then prevent any subsequent inputs on that open switch from having any effect for 200-500 ms (again, either ~200 or ~500, TBD by testing results). Essentially, a long debounce period is needed given that the switch is operated by the foot on a pretty hostile platform (lots of vibration, rider fatigue, etc. can all contribute to inadvertent multiple pulses).

For the proof of concept project, I decided to use a 556 IC (a double 555 timer). The attached schematic models two 555 chips because the simulator I used (the Java based one on the Web...falsted something or other) doesn't have a 556 component, but this should be fine.

The intended function is as follows:

-closing of switch sends a momentary logical low input to the first trigger (by discharging the capacitor, with the diode preventing a high spike) (this is to debounce the switch from electronic bounce/noise);

-first timer outputs 5V to a LED indicator and into the base of an NPN transistor, which in turns drives the 12V relay that will do the actual work (breaking power to ignition coil circuit...this mimics the function of the already existing manual "kill switch"). This impulse lasts ~ 60 ms with the R1 and C1 values in the circuit.

-first timer output also causes the trigger of the second timer to go HIGH. When the first timer expires after ~ 60ms and the output goes LOW, the falling edge triggers the second timer.

-output of the second timer feeds 5V (thus HIGH) to the trigger of the first timer, preventing the first timer from going LOW (and thus triggering). This lasts ~120ms with the R and C values in that circuit.

-the diode in that connection prevents first timer from going LOW when the output of the second timer goes low. Or does it?

This runs fine on a simulator, and I have it breadboarded but need to buy capacitors today to see if it works on the bench. But even assuming it works, I'd like to see if anyone has any thoughts on the wisdom or lack thereof of the design.

I also have a couple of more specific questions. I am not sure the circled resistor is necessary, it just seemed "right" for the two parallel branches going out from the first output to have matching resistors. Also, would it be preferable to also put a capacitor somewhere in the second trigger to ensure a very clean voltage drop when triggered (which apparently these chips like)?

Finally, does my second-output-into-first-trigger idea work to lock out the first timer regardless of switch state? And is the diode enough to prevent triggering the first timer when the second one goes low? (I opted to do this reverse-style lockout because I wanted to reduce the number of transistors in the circuit...initially I thought about using the first timer going HIGH as a trigger through a NOT gate for the second timer; the first timer would stay HIGH for 200ms and prevent retriggering, the second timer would drive the relay).

Thanks in advance for any help!
 

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  • QS Diagram - 556 Monostable Debounced plus.JPG
    QS Diagram - 556 Monostable Debounced plus.JPG
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We just had a thread along similar lines that can be found here. The idea being to trigger a one shot and prevent retriggering for a predetermined period. We seemed to gravitate towards using the RST and holding it low. Anyway, the thread may be worth a read.

Ron
 
Ahh, of course! Forgot about that RST pin! I don't mind a transistor in that spot, I just felt that using a NOT gate (via NPN transistor) was a bit tricky on the triggers to reverse the inputs there AND to get capacitor decoupling, and even that is probably my limitation and not the concept's.

Thanks!
 
Why debounce the trigger of a timer? Your scheme for preventing re-triggers definitely won't work. What's the point in preventing re-triggers for 120ms? Think about disabling the timer until some time after the switch is open again. Neither the circled resistor not the 100K one connected to it are needed. And as this is for automotive, having a clean, relgulated power that protects against load dumps will be essential.
 
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RallyeMax; Did you get this working with holding the trigger pin high? I got an e-mail from a responce to my post but cant see it on the form. Andy
 
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