BEAM robot IC swap

[Lucky-Luka]

Hi all
I was looking at this kind of photopopper
http://solarbotics.net/library/circuits/se_t1_1381.html
Is there a newer alternative to this kind of IC (MN1381-x)?
Thanks

 

[rjenkinsgb]

A Microchip TC54VCxx should work fine - the next two digits are the switching voltage, eg. the two are 4.3V and 4.5V switch respectively. They are available in multiple voltage thresholds from 1.4V upwards.

Add a resistor between the output and the transistor base, to reduce the chance of damage and limit the power wasted by the IC once the circuit triggers; try 1K to start with.

Also add a resistor in series with the collector of the PNP transistor; as drawn it's a rather poor circuit, as the two transistors will also short the capacitor and waste a lot of the stored energy when they turn on. 100 Ohms should improve it a lot!

https://eu.mouser.com/ProductDetail/Microchip-Technology/TC54VC4302EMB713?qs=gmCwDeenS6xrVPye%2Fu9vNA%3D%3D

https://eu.mouser.com/ProductDetail/Microchip-Technology/TC54VC4502EMB713?qs=9rDT7u5gGIrhLkTl5lROnw%3D%3D

 

[Lucky-Luka]

Why do you think that those two resistors will reduce the waste of power?

 

[rjenkinsgb]

1: The voltage detector IC output actively switches high and low, depending on the sense voltage.

Once the two transistors switch on, they "lock" each other on and will stay on down to probably something like 1.5V or so.

The voltage detector output will go high at the threshold, eg 4.2V, then go low again as soon as the voltage drops somewhat below that - so it will "fight" the collector of the PNP transistor now passing current to the base of the NPN transistor. That wastes energy in the capacitor and may damage the voltage detector.

Adding a series resistor to its output limits the current to safe levels.

A diode such as a 1N4148 would actually do the same thing but better still, preventing any reverse current in to the voltage detector output. (Anode to detector, cathode to NPN base).


2: The two transistors, once triggered, are directly shorting the capacitor in parallel to the motor and taking a large proportion of the stored energy - when the PNP one turns on, its just got the NPN base-emitter "diode" in series across the cap, with no current restrictions at all.

It's job is only to hold the NPN one on once the circuit triggers, so it can have a resistor in series with its collector to limit the current through it and the NPN base, leaving far more energy for the motor.

 

[Lucky-Luka]

I have noticed that under direct sun maybe the solar panel gives too much current and the motor starts to spin and never stops. Any suggestion to have the intermittent spinning?

 

[Lucky-Luka]

I have solved with a resistor in series with the solar panel output.

 

[Nigel Goodwin]

I think you've missed the point? - the whole idea of the beam design is to make best use of what power is available, and if the power is too weak. to gradually store it up, and then move a little using the stored power, then start over again. If the available energy is high enough, then it's supposed to run continuously.