my first B.E.A.M. bot attempt - question?

Hi there,

I am new in this forum. I was wondering if I could find help in this forum about my first beam bot project (a Symet) that I'm working on. I have some little problems that I don't understand quite well. Is this the right place to find help for these problems? Thanks.

Hi welcome to the forum!

Well what are the litle problems you have becuse we need to know what you need help with before anyone can help you

Good look with your robot!

Thanks!

This is the circuit I'm talking about :

**broken link removed**

It's a circuit that uses a flashing LED as a voltage trigger.

And that's the breadboard picture I used to put the components into my own breadboard:

**broken link removed**


This is what the book says about troubleshooting the circuit :

If you see the FLED actually flashing, then the solarengine resistor is too low for the solarengine to trigger the latch. Get a higher-value resistor, and swap it into the circuit. A typical trigger voltage for the FLED solarengine is about 3 volts for the green FLED and 2.6 volts for the red FLED. If the measured voltage on the power storage capacitor is higher than this, then the FLED is probably not getting power. If you don�t have a multimeter to test this, you can do it with a single white or blue LED. Just put this LED in parallel with the power storage capacitor (the long leg of the LED goes to capacitor �+�). If the LED lights up, then there is probably more than three volts in the capacitor.

I did some testing on my circuit and this is what I came up with :

- my solar cell puts out about 3V (sometimes less, sometimes more)
- the charge of the capacitors is about 3V each
- if I use a 1 to 10K resistor, as the book says, I can see the LED flashing
- if I use 22K, I cannot see it flashing anymore
- if I disconnect the motor, the LED flashes brightly (I guess the current then takes a bypass or something)

Are there other things you would like to know that I can measure/test?

Thanks in advance!

I have posted a reply but I cannot see it.

Edit : Maybe it's in moderation queue

Please use propper symbols, apart from looking 'playskule' it makes the circuit harder to understand.

Capacitors won't store enough power for the motor to stay running for longer than a couple of seconds, you need batteries.

I'm sorry about the schematic. Here is a proper one :

**broken link removed**


@Hero999: I know that the solar cells do not put out enough power to run the motor, but that is what the circuit is all about, I thought.

The circuit has a very interesting explanation:

The FLED is a integrated circuit that needs a minimum voltage to operate and flash the LED. While the LED is off the current through the FLED is very low. When the LED is on, a pulse of current passes through the FLED.
The solar cell charges the main capacitor until the voltage is high enough for the FLED to start flashing. When the FLED flashes, current flows through the FLED and the base of the PNP transistor and it turns on. Now current passes through the PNP into the base of the NPN transistor and it turns on. When the NPN turns on the collector which is connected to the motor and the 2.2K resistor goes low (to GND). This places a voltage across the 2.2K resistor which provides more base current for the PNP transistor which make it turn on even more. That is called positive feedback or latching of the circuit because both the PNP and NPN transistors remain on untill the main capacitor is discharged to less than 0.7V. When the capacitor voltage drops below 0.7V the PNP and NPN transistors both turn off because of the minimum voltage required to keep the base emitter turned on.

The 2.2K resistor sets the base current for the PNP to about 1 mA, which is amplified by the beta (current gain) of the PNP by 50-100.

That 50 mA of current flows into the base of the NPN and is amplified by about 10 to drive the motor. As long as the motor draws no more than 500 mA this circuit will work fine. If the motor current is much less, then increase the resistor value correspondingly. If the motor draws more current then uses a smaller resistor. Be sure to use a PN2222 or equal for the NPN for current greater than 200 mA.

Thanks for your time.

PS - I also did some more consistent measurements with the multimeter :

- solar cell + lead : 3.13 V
- capacitor + lead : 3.13 V on each one
- resistor + lead : 3.13 V / resistor lead on FLED-side : alternating between 0 & 3.13V
- BOTH motor leads : 3.13 V

is this normal?

Sorry for being such a noob, but I gotta start somewhere.

Hi hero, i think youve missed the point of the BEAM robotics, there not really built by electronics types - hence many BEAM schematics are drawn using pictures instead of symbols and they very rarely use batteries as they are meant to self sufficient - doing things like finding the stongest source of light.

Most of the ones i have built do little pulsed movements, they charge via the solar cell for a few secs, then move, then charge, then move....etc