John Fields said:
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!
Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone
is giving me fits!
Thanks!
---
[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND
JF
thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?
what am i missing?
Some basic knowledge of electronics, apparently.
As already pointed out, your 27 LEDs need 54V at the absolute minimum (at
room temperature -- they'll need more when they're cold) to operate. To
drive these directly from a chip requires a chip that can withstand 54V.
Your 555 can't do that. Your proposed alternative, the PIC, can't do
that either.
So you need to either (a) reconfigure the LED portion of your circuit so
it can run off of some voltage that a 555 can handle (and then cope with
the fact that a 555 probably can't deliver all the current you need,
unless you want 27 dimly lit LEDS), or (b) add some additional circuitry
to your circuit to switch the voltage that the LEDs need with the voltage
that a 555 has.
John suggested using an N-channel FET, which would be high on my list of
candidates, along with a big enough voltage source (probably much bigger
than 54V), and some device to limit the current to the LED string
(because LED's tend to want to be constant-voltage devices, but they need
to be constant current devices if you want them to light up and not burn
up).
Does it make more sense now?