I am working on a circuit that utilises hall effect sensors in a slightly unusual way and wondering if this would be seen as bad practice.

Please see the attached diagram which simplifies the circuit designed.

The idea is that the power to the hall effect sensor is controlled by the logic control circuit. Normally the hall sensor is powered and the output can change to reflect proximity of magnets.

Under certain conditions the logic control circuit would be used to remove power from the hall sensor thus preventing the signal from the sensor changing the state of the attached sequential logic. Since the sensor has an open collector output, I am assuming that output transistor merely switches off and will provide a high impedance between the output and ground connections thus causing a logic 'high' to be fed to the sequential logic when the sensor is disabled. Is this a dangerous assumption?

I do have a prototype circuit that seems to work but I am wondering if it is a fluke that it works at all or whether if I tried a hall sensor from a different batch it would cease to work?

I am wondering if this circuit can be relied on and over the full operating temperature of the device? Anyone know whether these devices be relied to power up / down gracefully without glitches / undefined states ?

My prefered choice would be to use additional logic circuitry to gate the output from the hall sensor before the input the sequential logic circuit thus allowing the hall sensors to be powered all the time. However this would come at the cost of board space and other points of potential circuit failure.

Any comments would be gratefully received

thanks
G

 

I would use logic to enable the advancement of the sequential logic. If board space is an ussue, use the space you are presently using for the stuff to turn on the sensor.

 

I agree with Brownout. Unless you need to save power it's normally better to use logic to control the signal rather than powering down the circuit.

I would think the small amount of added logic would be simpler, more reliable, and take less board space then whatever you use to control the sensor power.