Class project, Help me with resistor and resistor size

Question

I'm new to wiring with resistors. My goal is to use a bucket on a someone's head and have direction LEDs within the bucket to guide the person through a maze. So using a 9 V battery to power, then go to 4 switches (left, right, up, down, each button will be pressed to light up and guide the person). The LEDs are 3 V each. Can I get just 1 resistor and put it before any of the switches? If I do and 2 switches are used will it dim the LEDs badly? Or should I just use a resistor at each LED?

schematic

^{simulate this circuit – Schematic created using CircuitLab}

What is going on with those diodes? I assume you're connecting their cathode together to ground and their anode individually to switches. TLDR you want individual resistors for each LED, don't gang up their cathodes. This prevents any issues when multiple switches are turned on at once — crossroad, Feb 08 '22 at 18:13
how would i gather up the negitives back to the battery without the cathodes joining together? — Jeremy Stephenson, Feb 08 '22 at 18:16
Does this answer your question? Right way to connect leds to light up when pressing buttons — Elliot Alderson, Feb 08 '22 at 19:31
I noticed that your original schematic had the LEDs shown vertically, which was very confusing. There is a command in CircuitLab to rotate components - I suggest you revise your schematic to add the LEDs in the proper orientation, and add resistors in series with each LED. — Peter Bennett, Feb 08 '22 at 21:03

hacktastical · Answer 1 · 2022-02-08T22:41:45.483

1

Your diagram... is a problem. (Edit: I see you improved it.)

At any rate, you need to set the current though the LED to something that's bright enough for your use, but not so much that it fries the LED.

Typical LEDs will give useable illumination at 1-5mA, and can sustain up to 20mA without getting too hot.

How to set the resistance? There's 4 steps:

Look at your LED datasheet to find its forward voltage, or Vf
Subtract that Vf from your power supply. This is the voltage drop you need
Determine how much current you want.
Use Ohm's Law R = E/I to find the resistance.

Let's say you're using a super green (not yellow-green) or white LED, and you want to drive it at 10mA. These have a Vf of about 3V.

Voltage drop you need is then 9V - 3V = 6V. Since we're aiming for 10mA:

R = 6V/0.01A = 600 ohms

You could probably just use a 1k ohm and call it a day. That's bright enough for Buckethead and your battery will last longer.

If only one switch is on at a time you could use just one resistor. If not, then you need a resistor on each switch.

edited Feb 08 '22 at 22:41

answered Feb 08 '22 at 18:26

hacktastical

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so just get a 1k resistor for each Led then? – Jeremy Stephenson Feb 08 '22 at 18:43
Yes, that’s perfectly fine. That would allow more than one switch to be pressed at the same time (say, to indicate direction on 45 angles.) – hacktastical Feb 08 '22 at 18:58
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10mA with modern LEDs is really bright. It depends whether the bucket blocks out all light or lets a lot through on how bright buckethead's LEDs need to be. – Ian Bland Feb 08 '22 at 19:21
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For the DVRs I used to design, we’d settle on just 1-2mA for white, green or blue, any more than that was obnoxious in a darkened room. Reds would need a bit more. – hacktastical Feb 08 '22 at 22:10

score 0 · Answer 2 · answered Feb 08 '22 at 18:29

Your drawing is missing the LEDS so I am assuming they are in properly. Yes you should put a resistor in series with each of the LEDs if more then one is going to be one at one time. The resistor can be on either side of the switch or LED, but you only need one for each LED. If it is only one LED that will operate place the resistor in series with the battery.

Class project, Help me with resistor and resistor size

2 Answers2