# Beginner Facing a led and capacitor charging Problem.

#### foregp

Joined May 11, 2015
3
Hi,

I am a newbie to Electronics. I am stuck at following,

1.
I have a circuit

Battery -> Switch-> Capacitor -> LED.

When switch is on to complete (close) the circuit, Capacitor charges, meanwhile LED is flashing till capacitor is fully charged. The problem I am facing is that when I switch OFF the switch and turn it ON again LED does not flash because capacitor is still charged. I am unable to solve this (discharge the capacitor without losing the functionality of flashing LED only for a while when circuit is on).

I want to do above only using basic electronic components (Resistors, Transistors, Diodes)

2. Is there any electronic component like Transistor, which will pass the current if no voltage/current (absence of it) is given to base but will stop conducting if + or - voltage is given to base.

Thanks in anticipation, Eagerly waiting for replies.

#### DickCappels

Joined Aug 21, 2008
8,164
Please post a schematic of your circuit. What makes the LED flash?

#### ScottWang

Joined Aug 23, 2012
7,119
Yes, need the schematic that it will be better and you will get the answer quickly.

#### WBahn

Joined Mar 31, 2012
26,398
Is this a packaged blinking LED?

For your second question, you might look at depletion-mode MOSFETs.

Definitely need a schematic to know what you are working with and talking about.

#### foregp

Joined May 11, 2015
3
Thanks a lot for the replies. Here is the schematic.

#### ScottWang

Joined Aug 23, 2012
7,119
Where did you get this circuit?
How is the values of capacitor?
Maybe the capacitor is too big or you need to in parallel a resistor with capacitor as 1MΩ or some more.

What's your purpose for the led?

#### blocco a spirale

Joined Jun 18, 2008
1,546
Thanks a lot for the replies. Here is the schematic.

View attachment 85631
Your capacitor is the wrong way round; changing it won't fix the issue but it's good to get it right.

#### DickCappels

Joined Aug 21, 2008
8,164
Another hint: This circuit depends on luck to keep from killing the LED. Put a resistor in series with the LED to limit the peak current. High peak currents can kill an LED even though the average current and therefore the apparent "brightness" is low.

If you can describe the LED (what is the physical size and/or what the current rating is) and tell us the battery voltage we can recommend a resistor value.

#### neonharp

Joined May 10, 2015
12
Thanks a lot for the replies. Here is the schematic.

View attachment 85631
Hello
As some other posters have said , you need to have the polarity correct on the capacitor , you need a current limiting resistor in series with the LED , you need some way of discharging the capacitor between cycles . Not knowing the battery voltage , LED specs , or capacitor value , I really can't say what resistors you might need . You have somewhat of an R/C circuit here . Battery voltage would tell us what current limiting resistor to use . The time that you want the LED to stay on would tell us what capacitor value to use . A high resistance resistor in parallel with the capacitor ( high enough resistance that it would not pass enough current to keep the LED on after the capacitor has charged ) would discharge it between cycles . There again , we are taking about the timing of an R/C circuit as far as how long it it takes to charge/discharge . I don't know what times you are looking for . Here is a link about R/C circuits :

http://www.electronics-tutorials.ws/rc/rc_1.html

I hope that this helps

Cheers , take care

#### djsfantasi

Joined Apr 11, 2010
8,078
Here's a schematic showing what everyone is saying. I didn't have a pushbutton component, but it goes where you had it on your schematic.

#### AnalogKid

Joined Aug 1, 2013
9,639
Not quite. What he wants is a SPDT switch. The + end of the cap is tied directly to the + end of the battery. The - end of the cap goes to the switch pole (center or common terminal). One throw goes to the resistor going to the LED. The other throw goes to a low value resistor to the battery - end. In the "off" position, the resistor discharges the capacitor. Pick the resistor size depending on the minimum time the switch might be in the off position. In the"on" position, the discharge resistor is open-ended and the cap is tied to the LED and its resistor.

ak

#### neonharp

Joined May 10, 2015
12
Not quite. What he wants is a SPDT switch. The + end of the cap is tied directly to the + end of the battery. The - end of the cap goes to the switch pole (center or common terminal). One throw goes to the resistor going to the LED. The other throw goes to a low value resistor to the battery - end. In the "off" position, the resistor discharges the capacitor. Pick the resistor size depending on the minimum time the switch might be in the off position. In the"on" position, the discharge resistor is open-ended and the cap is tied to the LED and its resistor.

ak
Yes , that should work fine . But then , again , we don't know the timing we are looking for . Is the LED going to be operated every few seconds , once an hour , or once a day ? A push-button switch was shown on the original schematic . SPDT push-buttons are fairly rare unless you get into industrial duty controls , which are fairly expensive . The " bleeder resistor " approach would work fine , as long as you didn't need to cycle it real quickly .

#### AnalogKid

Joined Aug 1, 2013
9,639
Digi-Key has 12 different models between $0.68 and$1.54 each, one piece price.

ak

#### WBahn

Joined Mar 31, 2012
26,398
SPDT push-buttons are fairly rare unless you get into industrial duty controls , which are fairly expensive.
Really? Radio Shack had them (the stores that still exist probably still do).

#### neonharp

Joined May 10, 2015
12
Digi-Key has 12 different models between $0.68 and$1.54 each, one piece price.

ak
Hey thanks for looking that up , I guess I didn't do my homework on that one . maybe it's because I have been more accustomed to working with industrial controls .

Take care !

#### neonharp

Joined May 10, 2015
12
Really? Radio Shack had them (the stores that still exist probably still do).
Speaking of Radio Shack , they were a good company with good products , but they ran into a lot of financial problems over the years . I have a DX-150A communications receiver from the 1970's and a realistic weatheradio that I use to monitor NOAA , they still work great ! Yeah , most of their stores are now closed due to going bankrupt , and we all have to order all of our electronic parts online . My last order of parts ( some transistors and IC's from a surplus company in Mentor Oh ) the prices of all of the parts were good , they cost me about $20 , but shipping & handling was$9 ! Oh well .