Mims electronics learning lab

Thread Starter


Joined Apr 28, 2011
Hello, I just got the Forest Mims Electronics Learning Lab sold by Radio Shack. I've built the first circuit and it works, but I don't understand why, so I have uploaded a web page with a picture and some questions in hopes that someone here can clarify things for me. I'm sure it's very simple stuff for everybody here, so thanks in advance for any help. I hope it's correct procedure to use my own web space instead of taking up room on the allaboutcircuits server -


Joined Oct 2, 2009
I think you are way over your head here. Take one step at a time. No, one toe in the water first. You'll have to begin with how basic circuits work, R and C, followed by diodes and transistors. There are just too many sites on the net to list here. But don't give up. The journey will be well worth it.
I agree with MrChips, you'll benefit hugely from slowing down and understanding basics like the difference between voltage and current before trying to understand the functioning of a 555 timer circuit. Vol 1 of the book on this website will be a good read for you (I'm assuming you haven't already read it).

Once you've genned up a bit, a visual demo of what's happening in the 555 will make everything a bit clearer: Visual Demo link. The Doctronics page about 555s is also pretty good: Doctronics 555 Page.

As MrChips says, don't give up, when you can design your own circuits or even just understand how a circuit you've made works, it's extremely satisfying.



Joined Jul 17, 2007
Don't worry about using space on this server; if the image is too large, it'll tell you.

Also, it's best to simply put your text in the reply box rather than in the image, as it's a lot of extra work for people to have to re-type your original questions to reply to them.

I'm guessing that the current is flowing from the positive terminal of the supply to four different places: <snip>
Yes, that's correct.

I'm sure the 1k resistor limits the current going into the LED, the 4.7k limits voltage to pin 7, and the 10k limits voltage to pins 6 & 2 & the capacitor.
Resistors limit current, not voltage. Current passing through a resistor causes a voltage across the resistor. E=IR or Voltage = Current * Resistance.

Pin 1 on the 555 is ground, so I guess the voltage left over from whatever happens in the chip leaves from there.
For simplicity's sake, pin 1 (ground) is considered to be at 0v (ground potential) at all times. This is where the current leaves the IC.

Pin 3 is an output so the LED must be getting voltage from there, but I thought it was coming from the positive terminal of the supply??
Pin 3 is indeed the output pin of the 555, and it switches back and fourth between nearly ground and about 1.3v less than pin 8 while in astable operation mode (astable (free-running) multivibrator (square wave oscillator)). Pin 8 is the current source for pin 3, and pin 1 is where the current is sunk to. Transistors inside the 555 connect one or the other to the output.

When the capacitor has blocked the current enough then the input to the chip at pin 2 triggers the timer and current flows from pin 3. This seems odd to me because I thought that current was flowing from the positive terminal of the supply to the LED.
Well, capacitors DO block DC current. However, in this case the capacitor is being used as the "C" in an RC network. The capacitor charges from +6v via R1 and R2, and discharges via R2 through pin 7 (which is an open-collector current sink) and finally pin 1.

Pin 2 is the trigger input (roughly 1/3 Vcc), and pin 6 is the threshold input (roughly 2/3 Vcc).

When the charge on C1 has exceeded 2/3 of Vcc (6v*2/3=4v), the threshold limit, then the output pin 3 goes low, and pin 7 starts sinking current from C1 via R2, discharging C1. Current is also being sunk from the 6v supply via R1, which is why R1 needs to be 100 Ohms per volt of Vcc or higher.

When the charge on C1 has dropped below 1/3 Vcc (6v*1/3=2v), the output pin 3 goes high, and pin 7 stops sinking current, allowing C1 to start re-charging.

pin 4 is an input <snip>
Yes; pin 4 needs to be above ~0.8v, or the 555 will be in 'reset' mode and will ignore other inputs. This is usually accomplished by wiring pin 4 directly to the positive supply, or pin 8.

Pin 7 is used to discharge the capacitor?? how?
As explained previously, pin 7 is an open collector output. It is either sinking current (when the output is low) or it is electrically open (near-infinite resistance to ground) when the output is high.

The interval ends when the voltage at pin 6 is greater than at pin 5. How is this relevant, since the time it takes for the capacitor to charge is what's controlling the interval?
When the voltage on the cap exceeds the threshold limit, then the output pin 3 goes low, and pin 7 starts sinking current to discharge the cap via R2, as already explained. Changing the voltage on pin 5 changes both the threshold and the trigger voltages.

It is possible to cause the timer to cease to function if you set the voltage on pin 5 higher than the positive supply, or lower than GND. Decreasing the voltage on pin 5 will caue the frequency of the output to increase, as there is less time necessary for C1 to charge and discharge to the threshold and trigger limits.

I think there are two parallel circuits here, it's not a series circuit?
There are actually several subcircuits in parallel. National Semiconductors' datasheet on the LM555 has a schematic of the internals on the 1st page.
Go to http://www.national.com
and search for LM555.

Thread Starter


Joined Apr 28, 2011
Excellent, thanks gents, I'll start by looking for information about the simplest RC circuits. Believe it or not, I actually understood some of SgtWookie's explanation, so there may be hope for me :)