# Super Simple Circuit questions

#### Jacob Johnson

Jan 4, 2017
14
Yes, I'm a noob. I just need a couple answers that I can't seem to find otherwise.
I'm trying to make a really simple and cheap (focus on quantity) circuit to turn on LEDs for a set period of time at the press of a momentary switch (push button, lights light up for 3 minutes or so, then turn themselves off).
So I need to know what I need to purchase, and how to put it together.

#### Arouse1973

Dec 18, 2013
5,178
You need a retriggerable monostable, these can be easily made using standard logic gates and some timing components such as resistors and capacitors, you should be able to find lots of circuits on Google.
Thanks

#### Herschel Peeler

Feb 21, 2016
401
Yes, I'm a noob. I just need a couple answers that I can't seem to find otherwise.
I'm trying to make a really simple and cheap (focus on quantity) circuit to turn on LEDs for a set period of time at the press of a momentary switch (push button, lights light up for 3 minutes or so, then turn themselves off).
So I need to know what I need to purchase, and how to put it together.

Retriggerable? Why?
LM555 is okay and cheap.
Battery powered?
What LED did you have in mind?
Here's a starting point. With a 390K timing resistor I get about 2 1/2 minutes. With a 430K I get about 3 1/2 minutes. Specifics depend mostly on how close your 470 uF cap is.
Total cost for parts shown comes to less than $1.00. (P.S. After playing with the circuit, the diode is not needed. Replace it with a wire.) Last edited: #### Jacob Johnson Jan 4, 2017 14 Retriggerable? Why? LM555 is okay and cheap. Battery powered? What LED did you have in mind? Here's a starting point. With a 390K timing resistor I get about 2 1/2 minutes. With a 430K I get about 3 1/2 minutes. Specifics depend mostly on how close your 470 uF cap is. Total cost for parts shown comes to less than$1.00.

(P.S. After playing with the circuit, the diode is not needed. Replace it with a wire.)

View attachment 31235

Hey, thanks. At the very least, I can use your diagram.
So, I'm afraid I might not have been clear enough when I said I'm a "noob". I have minimal knowledge, at best, of electronics. I recognize a couple symbols in the diagram (resistors, capacitors, diodes, switch, and ground), but overall, it's really over my head (mostly your numbers). :/
I'm looking for a very simple and small circuit for about five or six small LEDs. With enough battery power to light them at full capacity for a good amount of time (many many cycles of approx. 3 mins)
It's possible I might be able to figure out what to do with your diagram (with help from my brother), so that's a great start. I appreciate the help.
Any further clarification would also be appreciated.

just which parts I need (exactly what to search for) and in what quantities. Aside from that, I'll figure it out.

Thanks, again,
Jacob Johnson

PS: Also thank you to Arouse for trying to help, but unfortunately, half your reply went directly over my head, and the rest was meaningless without the part I didn't understand. :/

#### Jacob Johnson

Jan 4, 2017
14
Oh, I should probably add that the circuit should be very compact and self-contained (ie: portable and completely encased)

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,760
Get a kit like this or like that?
Build it into a case of your own design.

#### Jacob Johnson

Jan 4, 2017
14
Get a kit like this or like that?
Build it into a case of your own design.
Idk, that seems a bit pricey for my propose. :/
Herschel's option seems more reasonable at a mere buck a pop. While those kits would greatly simplify the project (rather make it easier for me to do), I'm afraid it would turn out far too expensive, and tbh, those circuits seem a little excessive (I'm certainly no expert, but I don't think my circuit should require that many components (or maybe it just looks like more because of the green boards)).
Thank you, tho.

Last edited:

#### Jacob Johnson

Jan 4, 2017
14
I apologize if, by being so naive as I am, I am misusing the forums.
I would be asking a person like at an electronics store, but there aren't any out where I live. :/

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,760
If a kit is too expensive, you'll have to put up the circuit by yourself. See this example for example.

I apologize if, by being so naive as I am, I am misusing the forums.
You don't that's what we're here for. Only it's a bit difficult if on the one hand you don't want to spend the money for a kit but don't understand Herschel's schematic on the other hand.

For more than one LED the schematic needs to be modified. You may have to add a driver to power the many LEDs.
What battery voltage are you goinf to use? This makes a difference. With a low battery voltage, you will have to put the LEDs in parallel, each with its own current limiting resistor, bu the total current will be comparatively high.
With a high battery voltage you may be able to put the LEDs (or at least some of them) in series with only one current limiting resistor. The total current will be less.
Power will be the same in both cases.

For more detailed help we need more parameters:
- operating voltage
- type of LED (diffferent LED types have different operating voltages)
- current per LED (depends on teh type, too)

#### Herschel Peeler

Feb 21, 2016
401
Hey, thanks. At the very least, I can use your diagram.
So, I'm afraid I might not have been clear enough when I said I'm a "noob". I have minimal knowledge, at best, of electronics. I recognize a couple symbols in the diagram (resistors, capacitors, diodes, switch, and ground), but overall, it's really over my head (mostly your numbers). :/
I'm looking for a very simple and small circuit for about five or six small LEDs. With enough battery power to light them at full capacity for a good amount of time (many many cycles of approx. 3 mins)
It's possible I might be able to figure out what to do with your diagram (with help from my brother), so that's a great start. I appreciate the help.
Any further clarification would also be appreciated.

just which parts I need (exactly what to search for) and in what quantities. Aside from that, I'll figure it out.

Thanks, again,
Jacob Johnson

PS: Also thank you to Arouse for trying to help, but unfortunately, half your reply went directly over my head, and the rest was meaningless without the part I didn't understand. :/

Next step then. Attached is a data sheet for the LM555. It can be configured to create a string of pulses or a single pulse. Shown is the basic single pulse deign. Timing is controlled by the 470 uF capacitor and the 390K resistor. Pin 3 is the output. It goes high when the trigger input is taken low. How long it stays high depends on the R and C mentioned.
The output can drive over 100 mA high or low. So it can drive lots of LEDs.
For lower power there is also an 7555 that is CMOS. Same pin out and function but can't drive as much output current.

ak

#### KMoffett

Jan 21, 2009
723
Jacob,

Q1 is a P-Channel MOSFET. (label should have been NDP6020P)
With 9 V you could do 6 LEDs.

AK,
That Mosfet was used for a 3V battery circuit. Changing the circuit for a N Logic level Mosfet would improve availability and cost.

Ken

Last edited:

#### Jacob Johnson

Jan 4, 2017
14
Jacob,

Q1 is a P-Channel MOSFET. (label should have been NDP6020P)
With 9 V you could do 6 LEDs.

AK,
That Mosfet was used for a 3V battery circuit. Changing the circuit for a N Logic level Mosfet would improve availability and cost.

Ken

View attachment 31256
Ok, thank you. I think I've figured it out. This schematic is very helpful.

Thanks everyone for helping out so much.

--Jacob Johnson

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
The switch across the 1uF capacitor may not like the current from the capacitor. I'd put a 100 ohm resistor in series with the switch.

#### Jacob Johnson

Jan 4, 2017
14
The switch across the 1uF capacitor may not like the current from the capacitor. I'd put a 100 ohm resistor in series with the switch.

Do you mean between the switch and the leds/capacitor, between the switch and the capacitor/mosfet/resistor, or like this:

(yes, I drew the circuit on paper, yes, I screwed it up, at first)

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