Hey,

I'm planning to build a BEAM robot and found this smart circuit (from https://www.solarbotics.com/product/k-pp). However i'm not sure to understand how the photodiodes are used in the circuit.



It's mentioned in the manual that the one receiving the most light will trigger the voltage trigger faster, but wouldn't they need to be reverse-biased (cathode to positive leg of C1) for that? How does this configuration work exactly?

Thanks!

 

Hi td,
Welcome to AAC.
Recall that IR photo diodes also work in that reverse sense, their leakage current increases with increasing IR .
E

 

Hi td,
Welcome to AAC.
Recall that IR photo diodes also work in that reverse sense, their leakage current increases with increasing IR .
E

Thanks for the quick reply!
From my understanding, the photodiodes would act as current sources under light, with current going from cathode to anode. Wouldn't that decrease the voltage at the corresponding trigger input? I might be missing something obvious here.

 

Hi td,
I will over your link documentation, let you know.
E

 

hi td,
This is a clip from the datasheet:
2 - 1381C Triggers (one per Millerengine). These measure the voltage from the photodiode eyes.
The 1381 Voltage Triggers

I follow how they intend the PD to work, the voltage the PD produces is proportional to the IR intensity.
The PD creating the highest voltage will charge the connected 0.1uF cap, this voltage is applied to the 1381 voltage triggers the connected NPN transistor.

So it should work as they intend.

E

 

Photodiodes can operate in either photovoltaic or photocurrent mode.
The photocurrent mode is indeed much faster, but the current is minuscule. You would require a trans impedance amplifier. That is the reason they are operating in photovoltaic mode, it doesn’t require the ultra speed and simplifies the circuit.

 

Thanks for all your inputs!

I simulated the photodiode behaviour in LTSpice (using an optoisolator) and the voltage difference around the 0.1uF capacitor gets lower when the light increases.
Although I see on the component placement diagram that the left photodiode is used to trigger the left motor (and right for right). It make senses that if the left diode receives the most light, the left trigger voltage will be lower and the right motor will trigger sooner, making the robot turn left and following the light.

I ordered all the parts and will try to confirm this when I receive them.