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I have integrated a sensor to trigger a water pump to dispense water and sanitiser.

However, this sensor stays on for a minimum of 5 seconds after a person moves out range.

I have tweaked the time delay potentiometer to the lowest setting, and that's 5 seconds.

I want to be able to reduce it to just 1 second or TURN OFF as soon as a person moves out of range of the motion sensor, to avoid waste water.

How do I do that?

I understand that the sensor has a limitation. How can I circumvent it, or devise something around it to shut off before the default 5 second minimum delay?

HCSR501 datasheet.

This device has a BISS0001 passive IR switch that detects motion.

Look at the delay time 'minimum stating 5 secs adjustable (snapshot from the data sheet.) This is the one I want to reduce even further.

enter image description here

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Circuit diagram from the data sheet:

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Found an interesting link on Youtube that tells to short the R13 and R33 so that the delay time gets reduced. I shorted the resistors, but still no effect.

Video link is:

https://www.youtube.com/watch?v=juOtoUabyH8

enter image description here

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  • \$\begingroup\$ Please edit to include a link to the datasheet (not an Amazon or Ali-whatever advert page) and add in the device schematic. \$\endgroup\$
    – Transistor
    Jul 17, 2020 at 6:43
  • \$\begingroup\$ Thanks @JRE for the edits. \$\endgroup\$ Jul 17, 2020 at 8:44

2 Answers 2

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I have never used the BISS0001, but after looking at the datasheet I'd say you need to reduce R13 only. That is, lower R13 to maybe 1k, then turn down the delay adjustment. That ought to get you a shorter delay.

If that doesn't work, then I'd use a smaller capacitor in place of CY1. Use maybe half the given value - that should shorten the time to about 1.5 seconds (but put R13 back to its original value.) If that works, you can try a smaller value to get closer to whatever delay you want. Too small may not work at all, though, which is why I suggest a moderate change as a test first.

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  • \$\begingroup\$ Thanks @JRE. I could not do the entire soldering. These are SMDs and real tiny. I was able to add the 1K - but no use - I am not sure I can desolder and solder a small cap in place, The new circuit has SMDs that are hard to place in. Thanks so much for the reply. \$\endgroup\$ Jul 18, 2020 at 9:48
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Old question which got bumped in the recent activity feed, so the answer will probably not help OP, but could help others.

One option on this case, which does not involve changing the existing circuit but only adding stuff to it, would be to use a 555 timer in monostable mode. This way, when the output of the HC-501 changes, it triggers a pulse with a duration of your choice (configurable through the choice of one resistance and one capacitor).

The slight complication is that the 555 is level-triggered, so you’ll need to add a few components to make it edge-triggered to avoid the 555 repeating the pulse during the whole duration of the HC-501 longer pulse.

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