Using a 555 timer with a pull high trigger

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Joined Nov 4, 2015
I have two PoC units. Both devices make use of a standalone accelerometer (Bosch BMA250 Dedicated mode C).

The first device senses motion and interrupts the continuity between ‘voltage-free’ (almost) inputs on the radio. Interrupt pin on accelerometer pulls high which engages a relay via a bc547. (load: relay, 3v, 9.2mA)

The second device senses motion and alerts by beeping as well as sending a radio signal. Again, interrupt pin pulls high when in motion which makes the bc547 engage the buzzer circuit. (load: buzzer, 9v, 7mA)

The buzzer could do with engaging for a second or two after the motion signal pull low to make it more noticeable.

The relay could do with engaging for a second or two after the motion signal is pulled low because the debounce on our radio appears to be more severe than spec (it is ignoring brief movements).

I want to use a mic1555 in monostable mode to do this but it appears to require the input to be pulled low during alert state, whereas I have a pull high in that situation.

I got a smitt trigger inverter to act as a not gate to get the right input to the 555 but not working.

1. Is there a simpler way to achieve the delay?

2. Does a low power shmitt-trigger inverter have capacitor/resistor internally? The Texas instrument data sheet is vague/over my head.

Many thanks,

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Joined Jun 22, 2012
I would use an Npn transistor to invert the signal, put a 1k resistor to the base from the interupt pin, and a 10k resistor from the collector to positive supply, emitter to negative supply, and take the pulse from the collector to your 555 chip.

Option 2, put a diode in series with the buzzer and a 100uFto 1000uF cap across the buzzer to keep the supply on for a second or two, Or a capacitor across the relay coil.
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