Noob questions about timers and capactiors

Thread Starter


Joined Apr 13, 2008
Hi every1. I've just joined the forum and have already questions. I have basic knowledge of electricity and electronics, can read schematics, can solder.... and at this time I'm reading the all about circuits PDF's from this site.

Maybe some1 can point me in the right direction, here is what I need

basically switch A has to close only when switch B is in the middle position, number 2. Switch B has to switch positions like this 1,2,3,2,1,2,3 and so on, and has to be able to do this 10-20 times a second. I'm not sure how diffuclt this would be for a beginner and whether it is better to do this solid state or using mechanical switches/relays. Maybe a schematic based on the 555 timer?

and the second question is:

How can one isolate the capacitor so that it will only be able to discharge to the battery and NOT accept charge from it? For example I take the cap and charge it from a power supply and then hook it up to this circuit and the cap discharges fully to the battery.

many thanks in advance



Joined May 16, 2005
For your first question, it would be much much better to use solid state. Mechanical switches wear out quickly when run at the rates you describe.

For your second item, use a question. Note that "discharge fully" does not apply here. The cap will discharge only to the voltage of the battery, plus the drop across the diode.


Joined Jul 17, 2007
For your first schematic, you might take a look at a CMOS 4017 IC. It's a 5-stage Johnson counter with 10 outputs. Only one output at a time is high. Every time the clock is triggered, the next output in sequence goes high. It has a reset input that you could use to cut the count short.

Thingmaker3 mentioned a diode, which is what you use to allow current flow in only one direction. A standard silicon diode has a voltage drop (Vf) across it of somewhere around 0.6-0.7v, depending upon the amount of current flowing through it. Schottky diodes like the 1N5817 have a much lower Vf at low current levels (around 0.32v) that can make circuits like solar cell chargers more efficient.

See the attached circuit for how one might connect a diode in the circuit to permit current flow only from the capacitor to the battery. As Thingmaker3 mentioned, the capacitor won't be fully discharged; the battery voltage and capacitor voltage will even out, less the Vf of the diode.

Note that just a simple diode isn't enough to control the charge rate from the cap to the battery. You would need some sort of current limiter to keep from charging the battery too rapidly, or destroying the diode. Most battery chargers have a constant current circuit to limit the charge rate, and a voltage level detector circuit to determine when the battery is charged.


Thread Starter


Joined Apr 13, 2008
thanks folks

for the capacitor and battery setup I will probably use zener diode with a breakdown voltage around 12-13 volts

Thread Starter


Joined Apr 13, 2008
ugg, so depressing you call this noob and I can barely follow your question I must be like subsub noob
hey tech, I watch lotsa stuff on youtube and read the pdfs from this site. I kinda have an overall view of how circuits work and ideas for circuits but still lack the knowledge on how to pick the right values of components and how to connect them :) Step by step


Joined Apr 4, 2008
oooooooooh I know something for once... lemme answer!!!!!! yay!!!

If if you have window, use SPICE. If I understand it's pretty standard. a quick google search should find it


Joined Dec 27, 2007
I know, and I am not giving up, but man I can't wait till I understand something.
After 10 years of electronics, I still don't understand a lot of stuff! This is why I have a lot of respect for people like thingmaker, wookie, dave, etc, because it literally shows a life time of commitment to learning!



Joined Apr 26, 2005
Everyone has their favorite circuit simulators ...

Some like Linear Technology's

Some like the student version of Circuit Maker

Some like Tina ... with the free version being TI-Tina, located at TI's website.

Some use pSpice.

I don't know if Electronic Workbench, now MultiSim, has a student version.


Joined Apr 15, 2008
thanks folks

for the capacitor and battery setup I will probably use zener diode with a breakdown voltage around 12-13 volts
If you are planning on using the circuit that SgtWookie suggested, but replacing 1n5817 with a zener, you need to remember that a zener will conduct in both directions.