Newbie to electronics -- will a 1.5V relay switch work for this application?

Thread Starter

jag7

Joined Sep 9, 2020
3
Hi all. I'm a biologist and have a very basic understanding of electronics from my long-ago physics classes, and from doing repair on my home's electrical system. But much of the details go over my head. I have a very specific problem I'm trying to solve, and I think I've come up with a solution, but I'd like confirmation from people who know more than I do before I order the part.

I have an ebike that I built from commercially available parts, including a controller that contains the electronics. It receives inputs from a throttle and also a pedal sensor. The pedal sensor consists of a small ring of magnets attached to the base of one pedal, plus a hall sensor mounted to the bike. The hall sensor's signal is zero when the pedals are stationary or moving backwards, and ranges from about 1.2V to 5V (most of the time 2-3V) when the pedals are in motion. Current ranges from about 0.1 to 0.3 amps.

The controller also has an e-brake, which consists of two small wires, a hot and a neutral. When they are shorted together, the motor will not run, even if the throttle is on and/or the pedals are moving. This circuit is 5V, 0.44A.

I am moving from the US to Europe. In order to comply with EU law, I need the throttle to function only when the pedals are moving.

I *think* that I can accomplish this with a relay switch. The idea is that the coil of the relay would be wired to the pedal sensor, which is giving out a small voltage when the pedals turn, and the C and NC would be connected to the two wires of the e-brake. That way the switch is normally closed, which activates the e-brake, but when I pedal, the switch opens allowing the motor to run.

I found a company called Mouser that has about 100 different relay switches that have a coil voltage of 1.5V. My signal from the pedal sensor sometimes goes a tiny bit below that, but not often and only for a fraction of a second, so I think that a 1.5V relay might work, although if a 1V relay exists, that would be better. I found one that has a low coil current (23 mA) and figure that maybe a 100 A current at 1.2 V would still activate a 1.5V, 23 mA relay. That one is here: https://www.mouser.com/ProductDetail/Panasonic-Industrial-Devices/TXS2-L-15V?qs=YINDDaGsG3Hyab9HrGyr2w==. It's $5 plus $8 shipping, which seems like a lot for a switch, so if someone knows of a way to get it cheaper, I'd be grateful.

Is this a reasonable plan? Does anyone have any suggestions for other/cheaper ways to accomplish the same thing? I've read that transistors can act as switches, but I get the impression that they are typically normally open, and I need a switch that is normally closed, which seems to be pretty easy to do with a five-pin relay.

Thanks in advance.

JAG
 

sghioto

Joined Dec 31, 2017
1,453
Does anyone have any suggestions for other/cheaper ways to accomplish the same thing?
Trying to operate the relay directly from the output of the pedal sensor may be problematic. I would suggest adding a transistor to drive the relay coil. This will reduce the load on the sensor and allow higher voltage relays to be used based on what other voltages are available.
SG
EEE E-bike mod.png
 

AlbertHall

Joined Jun 4, 2014
10,397
I am moving from the US to Europe. In order to comply with EU law, I need the throttle to function only when the pedals are moving.
In the UK, at least, it is more complicated than that:
"Your steed is an “electrically assisted pedal cycle” (or EAPC, or ebike, or Pedelec) if: the bike has pedals that propel it; the electric motor won’t assist you when you’re travelling more than 25 km/h (15.5mph); and the power doesn’t exceed 250 watts."
https://www.cyclingweekly.com/news/product-news/electric-bikes-uk-law-234973
 

Thread Starter

jag7

Joined Sep 9, 2020
3
In the UK, at least, it is more complicated than that:
"Your steed is an “electrically assisted pedal cycle” (or EAPC, or ebike, or Pedelec) if: the bike has pedals that propel it; the electric motor won’t assist you when you’re travelling more than 25 km/h (15.5mph); and the power doesn’t exceed 250 watts."
Yes, the controller has a function to limit both power and speed, so I'm good there already.
 

Thread Starter

jag7

Joined Sep 9, 2020
3
Trying to operate the relay directly from the output of the pedal sensor may be problematic. I would suggest adding a transistor to drive the relay coil. This will reduce the load on the sensor and allow higher voltage relays to be used based on what other voltages are available.
SG
Thanks. I will study this and see what I can do.
 
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