# Why having two resistors (1k and 10k) in my circuit works?

I've put together a Vellman MK102 kit (Flashing LEDs) to try and get a handle on how circuit work.

I wondered why resistors where placed after the LEDs, and I learned that it's because resistors affect the overall current flow throughout the whole board (electricity being like a chain link rather than flowing like water through a pipe).

My board has both 1K and 10K resistor. How does that work? If electricity is resisted by 1K throughout the whole board how does it go up to 10K? Which has precedence and why are there two types of resistor in the board?

Can somebody explain to me how having 1K and 10K resistors in the same circuit works?

• Useful analogy: electricity is like water; voltage is like pressure, current is like, well, current; power sources are like pumps, wires are like pipes, resistors are like narrow pipes that constrain the flow. Capacitors are like chambers split by a rubber wall; inductors are kind of like heavy water wheels; diodes (and LEDs) are like... actually, I'm not sure what they'd be like. Some kind of a spring-loaded one-way valve? (And transistors are like... some weird water-actuated valves too? Maybe? Analogies only go so far.) Mar 17, 2015 at 14:09
• @IlmariKaronen +1 for "analogies only go so far" =)
– user20088
Mar 17, 2015 at 17:11
• @OP - that's an en.wikipedia.org/wiki/Multivibrator#Astable_multivibrator you have there - IMO, until you understand how the transistors and capacitors work here, you won't understand what the resistors do in such circuit. Also, electricity being like a chain link rather than flowing like water through a pipe - who the hell told you that?! The last analogy I'd use is "chain link" analogy for current... actually, en.wikipedia.org/wiki/Hydraulic_analogy makes a lot more sense. You based your question on false premises.
– user20088
Mar 17, 2015 at 17:13

electricity is resisted to 1K throughout the whole board

Not the whole board - just all the way through a particular path. Both water and the chain belt work as metaphors, but we're not used to chains splitting and recombining.

Recall that electricity flows in a complete circuit: a loop through the power source.

There are multiple complete circuits that current can take through the board, for example:

That path goes through the LED, resistor, and transistor. How much current flows depends on how "on" the transistor is. The resistor is there to limit current when the transistor is "fully on".

Compare:

The blue path goes through the variable resistor, which has a 10k resistor in series with it so behaving like a variable resistor from 10k to 260k. It also goes through the base of the transistor, tending to turn it on.

In both cases the resistors limit the current - to different values of current.

The actual operation depends on variable current flow in the capacitors as the transistors turn on and off; the easiest way to do this is to put it into a simulator like Falstad and watch the result. Astable multivibrator in Falstad. That example uses different values and so oscillates at a different frequency. Try varying them.

• The only missing fact, I think, is that the currents flowing through different subcircuits simply add if they flow through nodes that are shared. So the battery will "see" the sum of the red and blue currents. In this case, the currents are both positive, so they add up. Mar 17, 2015 at 17:43
• Thanks. It's still early days for me but that explanation helped a lot. Mar 17, 2015 at 18:59

The 1K resistors limit how much current flows through the LEDs so they don't blow.

The 10K resistors limit how much current flows into the capacitors, setting the "time constant" of the charging of the capacitors.

$$\tau = RC$$

This combination of resistor and capacitor is what sets how fast the LEDs flash.