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How to read Robot Schematic

badreligion9265

Mar 12, 2013
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Hi I'm new to the forums and was hoping someone could help me learn a thing or two about reading schematics.

I recently purchased and assembled a "Velleman Micro Bug" robot kit. The instructions provided include a schematic of the robot's circuitry. I want to be able to understand it all eventually. So I figured I'd provide a picture of the schematic and ask questions about certain parts of it and see if you guys can enlighten me on how the circuit is actually working.

Here is the schematic:http://www.esr.co.uk/manuals/manual_mk129.pdf

Now my first questions pertains to the left part of it. There is switch labled "SW2" which has two positions. My question is about the flow of current when the switch is in its depicted position. Could someone just explain to me how the flow is interacting with all the components just on the left side, I'm not yet concerned with learning the entire flow I just want to see if what I think is happening is actually the case.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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When the switch is in the position which opens the circuit, no current flows. When it is in the position which closes the circuit, current can flow.
 

badreligion9265

Mar 12, 2013
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Well that much I figured, but I was hoping for a more detailed explanation that describes the flow of current, and how that flow is interacting with the individual components in the circuit. An example of the type of explanation I'm seeking is as follows: "With the switch in the closed position current flows into Capacitor C2 which then stores the charge and lets it out in pulses towards the T1 transistor which then allows current to flow towards the R1 resistor....." Something along those lines. I have trouble grasping the full extent of the flow and its direction, and how all the components are using it and where it is going based on how those components direct it.
 

Osmium

Jan 28, 2013
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You're talking about the components to the left of SW2?

The two transistors T1 and T2 plus the associated capacitors and resistors form what is known as an "Astable Multivibrator". See http://en.wikipedia.org/wiki/Multivibrator for a detailed explanation. Briefly, this is a type of oscillator - a circuit which produces a time varying signal - in this case, a "square wave" at the collectors of the transistors. The transistors alternately switch between the "ON" state (low voltage at the collector) and the "OFF" state (high voltage at the collector). When T1 is "ON", T2 is "OFF" and vice versa.

Switch SW2 is used to select the signal from either transistor collector.
 

(*steve*)

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The first part of the circuit is an astable multivibrator.

I recommend you look that up in wikipedia because it will give you an explanation far better than I'm prepared to type right now. It's a lot like a see-saw though.
 

(*steve*)

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Now you know what the astable multivibrator does, you will note that the rest of the circuit is comprised of 2 very similar (indeed identical) parts.

These other parts control a motor (and use an LDR for some reason). They only operate when the input from the multivibrator is high.

The switch (sw2) selects the second of these to operate from the same output as is used by the first, or the other one.

The upshot of this is that in one position both motors get activated at the same time whilst in the other they are activated alternately.

Incidentally, the motor needs to have a diode across it to prevent damage to the transistor that switches it on and off.
 
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