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Topic: Failsafe- 50% max PWM (Read 1 time) previous topic - next topic

edmcguirk

#30
Mar 07, 2020, 04:01 am Last Edit: Mar 07, 2020, 04:02 am by edmcguirk
I think I will go back to the 555 timer used as a watchdog.



1 - Transistor Q2 controls the mosfet directly and is controlled by the arduino. Low signal is 'on'.

2 - The 555 timer triggers when the arduino signal goes low and at about 12mS the capacitor will reach the threshold and reset the 555 timer which will ground the mosfet gate. If the arduino low signal remains under 10mS, the 555 never resets.

3 - The ground signal from Q4 will also be used to ground the mosfet gate of the other pump (which will have a duplicate 555 watchdog) and illuminate an error LED.

4 - The default with no arduino signal is 'off' with the mosfet gate grounded.

Did I make a mistake? Anything else I should look out for?

edmcguirk

#31
Mar 07, 2020, 04:34 am Last Edit: Mar 07, 2020, 05:35 am by edmcguirk
The gate voltage is a little ugly but it seems to work well with a couple fewer parts.



edit - Probably needs a pull up resistor on Q2?

Or if I can get a logic level mosfet that can handle the load...



edit - Probably needs a pull down resistor on the gate?

dougp

I am not under any specific constraints. I just prefer to use fewer components taking up less space on the circuit board and I would like to limit the design as close as possible to generic parts cheaply available. My guestimate is that if the part is commonly available on ebay, then it could be considered a generic part.
There are hardware WDTs.  Perhaps you could adapt one to your need.
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JohnRob

To be honest I've not though through this fully and its been some time since I used a 555 timer, however;


I know you can control the output dutycycle by modifying the voltage on the control pin.   I don't know if the range is enough for your needs.

The analog output of an arduino is really a PWM signal of ~1000 Hz.  It would be easy to filter it and divide it to a lower voltage to control the 555 control pin.


Even if the above does not work out for you,  you should use an LMC555 as opposed to the standard 555.


The standard 555 design has a shoot-through issue in the output stage, where under some conditions both the upper and lower output transistors are on at the same time.  Could cause spikes to appear on your power supply.

The CMOS version (LMC555) does not have this issue and generally uses significantly less power.

John
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edmcguirk

I know you can control the output dutycycle by modifying the voltage on the control pin.   I don't know if the range is enough for your needs.

The analog output of an arduino is really a PWM signal of ~1000 Hz.  It would be easy to filter it and divide it to a lower voltage to control the 555 control pin.
That's interesting, it would be a completely different way for me to generate the 50hz PWM power for the vibratory pump. I don't know if it would be an improvement unless I could see how it would work. I did a quick search but did not find an example of keeping a constant frequency while varying the duty cycle by changing voltage on the control piin of a 555.


Even if the above does not work out for you,  you should use an LMC555 as opposed to the standard 555.

The standard 555 design has a shoot-through issue in the output stage, where under some conditions both the upper and lower output transistors are on at the same time.  Could cause spikes to appear on your power supply.

The CMOS version (LMC555) does not have this issue and generally uses significantly less power.
Given the 50hz frequency and the hysteresis of my circuit, I don't think I should have a shoot-through problem. Since my 10v supply is feeding only the 555, the mosfet gate, and the arduino voltage regulator I don't think I need to worry too much about noise. However, if I find a good logic level mosfet, the LMC555  sounds like a good choice when it's all running off the arduino 5v supply.

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