Hi guys,

been doing some reading but can't seem to find a simple solution to this problem, so hoping someone can help.

I have a mobility scooter (24v electric system).
Forward/reverse is controlled using a potentiometer.
I've measured the resistance which is 2.5kOhms at rest and then between 0.2kOhms and 5kOhms when turned.

What I want to do is create a reversing beeper/buzzer that will sound as soon as the throttle pot is moved i.e. when resistance across the throttle pot is >2.6Ohms

I can see some sample circuits for the reverse beeper but how can I easily turn it on when the throttle is touched?

Any advice appreciated.
Thanks in advance.

 

With the negative lead of your DVM (Set to the lowest DC range above 24 volts. This will probably be 200 volts) connected to the negative of the battery and the positve lead to the slider on the pot note the voltage readings at the stop position and the full speed forward and full speed reaverse. Also note the reading at the minimum reverse speed position. The readings will probably be less than 24 volts. If so set your DVM to the 20 volt DC range. Post the results.
Edit. A possible alternative to using the voltage from the pot would be to use the voltage to the motor. This probably uses PWM speed control so if you connected the buzzer via a diode with a smoothing capacitor across the motor (In the correct polarity) The capacitor would probably still charge to the full 24 volts even at the lowest speed. You would then only need to connect the buzzer across the capacitor. If the buzzer was less than 24 volts rating it would need a voltage regulator or series resistor of a suitable value.

Les.

 

Hi Les,

thanks so much for your reply.
I've done the measurements as suggested which are as follows:

Stop: 2.4v
Full Reverse: 4.6v
Full Forward: 0.3v
Min Reverse (just touching throttle): 2.6v
Min Forward(just touching throttle): 2.3v

I had thought about using the feed to the motor however initially I was against this as it's all external and therefore exposed to the air/water etc. I thought that using the throttle pot would mean I would do everything within the enclosed tiller.
Potentially an easier solution though that you have suggested.
If I was to do this, how would I calculate the size capacitor to use?

As a side note, I see you can purchase reverse buzzers that take a constant feed and output a pulsing beep sound. I assume they have a signal generator built in. Anyway, that would mean that potentially connecting this straight to the motors (via a diode and cap) would be a very quick solution.

 

I've still not managed to solve this so if anyone has any other ideas or suggestions I would appreciate hearing them.

So far I purchased a voltage smoothing circuit (just to try before building something similar): https://www.ebay.co.uk/itm/190814696793
And also a reverse buzzer that has a signal generator built in (again just to simplify thing initially): https://www.ebay.co.uk/itm/221106607119

Still can't get this working though. Seems the voltage regulator cannot cope with the variations sent to the motors.

I feel like going back to my original approach of using the throttle potentiometer as an on/off switch but not sure how to do this.

Thanks again!

 

It should be possible to do what you want using an LM431 adjustable zener regulator, an LM393 comparator and some other components. The link to the buzzer does not give any information on the current it requires. It also looks like there is only one wire to it and it gets the negative from the metal panel it is screwed into. If this is the case we need the switching transistor in the positive supply to it. I need to know what current it requires before designing the circuit.

Les.

 

Hi Les,
Thank you again for taking the time to reply.
If my understanding is correct, the regulated (smoothed) voltage will need to be less than the max.
So as it's a 24v system, I would be better building a voltage regulator with 24v in and 12v out.
If so I can replace the reversing buzzer with a 12v version. I have found one that draws 40mA.

With regards to the existing buzzer I have purchased, I can't measure it right now as not at work.
I think I remember it saying 99mA on the box but not sure. I can't find any others on the web with the current required shown.
I need to go to work later today so will try to measure it then.

Oliver

 

I was thinking of something like the attached PDF file. I have used a zener diode to drop the 24 volts down to 15 volts rather than a voltage regulator as it is more to filter out any high voltage spikes on the 24 volts. A voltage regulator could be damaged by spikes.

Les.