Re: Simle LED driver

Allen Bong wrote:
Hi,

I have a simple 555 delayed timer and I wish to light up a LED when
the timer starts and a relay off. At the end of the interval the LED
should be off and the relay on. I made a simple circuit as follows:

+------------+---+---------+---+------------+---------+--
VCC
| | | | | .-. |
| | | .-. | | | |
.-. |4 |8 | |<-+ | |470 |
| | .-------. | |1M '-' |
| |10k | | '-' | |
'-' | |3 | | |
| | |------------+ | |
| 2| 555 |7 | | led V -> |
o----------| |--o----o | - |
| | |6 | | | | | \
|+ | |--+ | | +---+ |_ o o
=== '-------' | | | | )|
/-\33u |1 |5 47u|+ | ___ |/ | )|
relay
| | --- --- +--|___|--| | _)|
| | ---10n /-\ 10k |> | +--
| | | | | | |
GND GND GND GND | | |/
| +-|
| |>
| |
| |
+-------o
|
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

But the LED stays ON at the end of the interval when the relay is
switched ON. I have to add an iverter and modified the circuit as
below:

VCC -----o---------o---o------------o---------
o------------o
| | | |
| |
| .-. | .-. |
\ .-.
|8 | |<-+ | | |_ o
o | |
.------. | |1M 10k|
| )| | |470
| | '-' '-' )|
relay '-'
| |3 | |
_)| |
| |------------+ |
| |
| 555 |7 | | |
| LED V ->
| |--o----o | |
| -
| |6 | | | | | |
\ |
| |--+ | | +---+ o---|

O------

'------' | | | | | |/
|5 47u|+ | ___ |/ | |
4069
--- --- +--|___|--| | |
---10n /-\ 10k |> | +-+
| | | | |
GND GND | | |/
| +-|
| |>
| |
| |
+-------o
|
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Is there any way not to light up the LED when the relay is switched on
with just the two transistors?

Allen


You can get a lot simpler:

+12 ------------------+------+
| |
[Relay] [D2]
D1 | |a
555 pin3]----+---|<---+------+
| a
[470R]
|
[LED]
|
Gnd

If you want to use a transistor driver for the relay:

+12 ----------------------+
|
/e
[555 pin3]---+---[1K]---| PNP
| \c
[470R] |
| +------+
[LED] | |
| [Relay] [D1]
| | |a
Gnd ---------+------------+------+

In either circuit, when the 555 is "on" there is + at
pin 3. That lights the LED, and keeps the relay off.
When the 555 is "off", there is - at pin three. That
keeps the LED off and the relay "on" (energized).

Ed
.

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