0
\$\begingroup\$

total noob here. I'm trying to figure out why an LED in a small circuit isn't getting to full brightness, and how to fix that.

It's a simple, pre-bought circuit, seen here http://postimg.org/image/iab6po9c1/ - originally taken from a basic led bike wheel valve light. Circuit like this:

Battery (3 x AG10) > Vibration Switch > LED > Battery.

(before you ask - http://www.electrodragon.com/product/vibration-switch-sensor-sw-18020p/ - but I've no idea of the exact specs of mine, just that I have a few different ones, and they all act the same way).

When the LED (any LED, I also tried a few) is connected directly to the battery, or I manually press the spring against the contact in the vib switch, it lights up at normal brightness.

However when the vibration switch is, well, vibrated and switched, I only ever get around 50% brightness from the LED. I assume the switch only makes contact for a very short period so doesn't let enough power through or whatever. Either way, it's dull.

I'm not sure exactly what to use, but I've seen similar stuff that puts a transistor or 555 timer between the battery and LED, uses the switch to activate the transistor or 555, that then pulls power from the battery and activates the LED:

Battery > Transistor > LED > Battery

| > Vib. Switch >|

Any ideas if this will get the battery to full brightness? If so, what type of transistor would I need (I got as far as NPN - don't wanna use 555 as it's be too big and complex)? And/or is there anything I'm missing (caps, resistors - trying to keep it simple - I have about 5mil of space and would prefer not to even solder). Thanks :)

\$\endgroup\$
  • \$\begingroup\$ It can be done using monostable multivibrator with two transistors (and some resistors and capacitor). Stable state would be off and time constant should be very short (depending how long you want it to be on after you stop shaking). \$\endgroup\$ – Darko Mar 9 '16 at 20:49
  • \$\begingroup\$ A 555 is fairly small. And you only need two resistors and two capacitors. A smaller option would be a single chip microcontroller. \$\endgroup\$ – Passerby Mar 9 '16 at 22:01
1
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

This is the simplest solution that might work, depending on battery resistance.

As you have deduced the brightness is reduced because the LED is blinking on and off at a higher frequency than your eye can perceive. A capacitor in parallel with the LED will charge up while the switch is closed and power the LED briefly while the switch is open. Something in the region of 10 to 100 uF should suffice. The voltage rating of the capacitor just needs to be >= 5 V. You should really solder it in.

Aside: LEDs require current-limited power supplies. Usually a resistor is connected in series between the power supply and the LED. Since your LED is powered by measly button cells with a high internal resistance the current is limited by the battery. If you were to connect the LED directly onto a power-supply with constant 4.5 V it would probably die.

\$\endgroup\$
  • \$\begingroup\$ Thanks - I tried 16v 47uF and 100uF caps - slightly brighter, mainly as it stays on longer and fades out (half a second). Still not full brightness - ie. point at a wall, short the vibration switch, I get a nice bright circle on the wall. activate by normal vibration and it's no where near. Just so you can see what I mean: youtube.com/watch?v=msRxWmekFaQ - that's me shorting the switch (pushing the spring against the contact), then just vibrating it to basically do the same thing. no cap, but with cap isn't massively different compared to shorting it, just stays on longer. \$\endgroup\$ – pepsi_max2k Mar 10 '16 at 12:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.