# Minimal Hall effect sensor circuit to keep something off after certain RPM achieved

I have a somewhat limited understanding of electronics and circuits. I'm trying to build a circuit that will keep a transistor in an off state after a given threshold of RPM is achieved using a A3144E Hall effect sensor.

For example, anything let's say that is below 500 RPM, I would like the transistor to be in an on state, and anything above 500 RPM, the transistor should be in an off state.

How could this be achieved using minimal and common parts without the use of a microcontroller, etc.?

You can do this with an LM2917 (or 2907) and a few support components. Have a look at page 9 of the PDF. The transistor state is inverted from your requirements, but that's easy to fix with another transistor. simulate this circuit – Schematic created using CircuitLab

Without actually building and testing it myself, and given that it's 20 years since the only time I've used an LM2917, I'm not going to promise you that this circuit will work - but based on the datasheet it should. It should cover from about 220rpm to 900rpm; changing the upper and lower revs is just a matter of changing the resistors.

The rpm is set by C1 and (R1+R2). R2 is adjustable, allowing you to set a resistance of between 33K and 133K.

The relationship between frequency, resistance and capacitance is: F = 1 / 2RC

F is rpm / 60, so:

• rpm / 60 = 1 / 2RC
• rpm = 30 / RC
• Using a 1uF capacitor (as I did in the schematic):
• rpm = 30,000,000 / R

So for a 33K resistor, rpm = 909 and for a 133K resistor, rpm = 226. Smaller resistor values give higher rpm's.

The normal behaviour is that the (internal) transistor switches on when the frequency is above the set point. Q2 (and R5) flip that behaviour - Q2 switches off when the frequency is above the setpoint.

• I'm a bit lost what else I would need. I'm trying to figure out what components I would need to make the RPM (frequency input to LM2917) adjustable for testing purposes. Could you do a rough schematic? Oct 9, 2014 at 3:52
• @troop231 Not clear... do you mean that you want to be able to adjust the threshold, to higher or lower than 500rpm? Oct 9, 2014 at 3:58
• Yes, just for testing, and also: Would it be a combination of figures 19 and 20 in the linked PDF to build what I need, except for the transistor change you mentioned in your answer? Oct 9, 2014 at 3:59
• Just figure 20. Figure 19 is for a voltage output, which is not what you want. Oct 9, 2014 at 4:02
• There appears to be a problem with using the LM2917 for what I want. It says the minimum voltage should be around 12 volts, and my system will use a 12V battery. Doesn't leave much room for error when the battery discharges slightly. Are there other chips that work on a lower voltage that do something similar? Oct 9, 2014 at 4:07