# How does the circuitry work in the Symet robot?

I ordered this Symet robot kit and it only includes instructions on how to build the thing instead of actually teaching you exactly why and how it works. I even ordered this book that walks you through how to create it again but again does not teach anything about how the circuitry works. So stinkin' lame!

The circuit diagram on the top is how the book teaches you to build it and the diagram below is how the kit teaches you to build it.

The top one uses two different transistors and a FLED while the bottom uses 3 different transistors, an additional tiny capacitor and a diode.

The "robot" works very simply, storing some power in the larger capacitors then releasing it to the robot, turning the motor in bursts. My question is, how is this circuitry working? How are the two (and three) transistors working to make this happen? Also, what are the big differences between the two and why is one better than the other? Any and all details of how this is precisely working would help me tremendously!

• – pjc50 Mar 6 '15 at 9:51

I agree with your observation that simply giving instructions on how to connect circuit components together with no further explanation does not help understanding how the circuit works. And I applaud your effort to learn the why and how.

My answer is going to focus on the first circuit because, honestly, I couldn't find any information on a "1381" transistor. Are you sure it's a transistor and not something else?

The 2-transistor circuit looks like this:

simulate this circuit – Schematic created using CircuitLab

Let's start from the end (making the motor turn) and work our way backwards. In order for the motor to turn on, current will have to flow through the motor. If you look at the schematic above, there are two current paths through the motor: one is straight down through Q1. The other goes through R1 and the LED. Presumably, the 1k-10k resistance of R1 will prevent any substantial current to flow in that path, so it won't generate enough electromagnetic energy to physically rotate the motor. Therefore, the only realistic current path to rotate the motor is through Q1.

Q1 is a NPN transistor, so a positive voltage must be applied to its base pin to turn on the transistor (typically, that voltage is ~0.7V). The base of Q1 is connected to the collector of Q2, so Q2 controls whether Q1 can turn on.

Q2 is a PNP transistor. In order for a PNP transistor to turn on, there must be a path for current to flow into its emitter and out of its base. Looking at the schematic, current can flow into the emitter of Q2, out the base, and through the diode. When this happens, the LED will light up, current will flow from the emitter to the collector of Q2, Q1 will turn on, and the motor will rotate.

EDIT 1: The emitter and base of a PNP act as a diode where the emitter is the anode and the base is the cathode. In fact, the arrows that are drawn as part of the BJT symbol represent that diode behavior. The electrons are trying to get from one side of the capacitor to the other, so whenever a path is available, they'll take it. In this case, the path is from the plus side of the capacitors*, through the "diode" of Q2, through the LED, and into the negative side of the capacitors. When the LED blinks off (I assume this is a self-blinking LED that turns itself off internally), that current path is cut off and the electrons can't get from one side of the capacitor to the other.

So ultimately, the blinking LED is controlling when Q2 is turning on and off. Q2, in turn, controls when Q1 is turned on and off. And Q1 controls when the motor energized or not. So when the LED blinks off, so does everything else.

The capacitors are there to collect and store the energy from the solar panel over time. This is because, presumably, the solar panel isn't large enough to directly run the motor by itself.

*Strictly speaking electrons flow from the negative potential to the positive, but electrical engineers don't like to think that way ;).

• Guess I was wrong about the 3rd transistor, it's a voltage trigger - solarbotics.com/product/1381 – Michael Rader Mar 6 '15 at 9:54
• A couple of things I still don't understand. "current can flow into the emitter of Q2, out the base, and through the diode. When this happens..." -- When and how does this happen? And how does the LED blinking off cause Q2 to turn off and then on when it blinks on? – Michael Rader Mar 6 '15 at 22:56
• @MichaelRader, I've added an edit to the answer that I hope addresses your question. If it's still not clear, post another comment with the specifics of what you're confused about and I'll try my best to answer. – Dan Laks Mar 6 '15 at 23:53
• Ok It's def becoming more clear, thank you! I think I may have had a false understanding of the way current works. So current isn't waiting right outside of the LED for it to turn on. When it is off there is no current at all flowing anywhere because there is no complete circuit, right? – Michael Rader Mar 7 '15 at 0:50
• Yep, you're getting it. Think of current as water in a pipe that is completely full of water (no air pockets). None of the water can flow unless all if the water is flowing in the same pipe. If one part of it gets blocked off, it all stops (caveat: oversimplified analogy, but works well for a high-level understanding) – Dan Laks Mar 7 '15 at 1:19

A VERY crude and simple design - BUT, if it does the job intended and the price is OK then good luck to it.

2 transistor circuit:

2N3904: When transistor is on top and bottom leads (Collector and emitter) are electrically joined and current flows from capacitors via motor and transistor.
2N3906: When this is on it turns the 2N2=3904 on via its middle control lead = Base.
The resistors pulls the 2N3906 base high and turns it off.
When voltage is applied to the flashing leg it flashes - fancy that :-).
When the LED tuns on it pulls the 3906 base low and turns it on which turns the 3904 on which drives the motor.
When the LED turns off the motor turns off.

3 transistor circuit:

Actually 2 transistors and a "reset generator" IC.

THey call the 1381 a "trigger diode".
It's "usual" name is a reset generator or voltage monitor (or similar). Its usual use is as a low or high voltage warning monitor but it is well used here. The 1381 produces a high output level on pin 1 when the voltage between pins 2 & 3 exceeds a preset voltage - and it removes the output when the input differential has fallen by a certain amount. Here the IC "turns on" when the capacitors (and so its trigger input) reach a certain voltage and turns off when the capacitors voltage has fallen by a set amount.
They use a 1381E. The E specifies turn on of 2.2 to 2.4 V and turn off when Vcap has fallen by between 50mV and 300mV. The large ranges mean that different robots will "pulse on" for different periods - but all will probably work well enough.

Operation of the 3 "transistor" circuit is similar to the 2 traansistor BUT the 1381 is used to turn the 3906 on when the input voltage is high - y when the solar panel has charged the capacitors to 2.2 to 2.4V and off again when the voltage falls 50 to 300 mV.

So the 2 transistor cct uses a flashing LED to cause on/off pulses. The 3 transistor cct uses capacitor voltage level to turn the motor on and off.

The 2 transistors both have connection E B C when viewed from left to right.
The 3904 is a "NPN" type - C is more positive than E during operation. The 3904 is "PNP" type - C is more negative than E during operation.

When B is at E potential the transistor is off.

As B is moved towards C potential the transistor turns on when Vbe is ABOUT 0.6V.

1381 datasheet

Their 1381 overview: https://solarbotics.com/product/1381/

• Thank you so much. Still trying to read and understand these answers to see which best answers the question. "The 2 transistors both have connection C B E when viewed from left to right." don't you mean EBC? - en.wikipedia.org/wiki/2N3904 – Michael Rader Mar 6 '15 at 9:51
• @MichaelRader Whoops yes - EBC sorry - will correct it - looking at the diagram I can't imagine how I managed that - I usually use the very good BC337 / BC327 which are CBE but the diagram is obviously EBC. – Russell McMahon Mar 6 '15 at 10:31