On Tue, 27 Dec 2005 07:35:17 -0600, John Fields
wrote:

On Mon, 26 Dec 2005 18:15:48 -0600, John Fields
wrote:


Aarghhh!!!

There's a gross error in the circuit which I'll fix tomorrow...

Hint: Vbe of the NPN won't allow the MOSFET to turn on, duh...

---
KISS works best...


+V
|
+-------+
| |8
[Rt] +---+---+
| 2|_ Vcc |3
IN---+----------|--O|T OUT|-- OUT
| | 6| |
| +---|TH |
[1000] | 7|_ 7555|
| +-----+--O|D |
| | +| | GND |
| E [Ct] +---+---+
+--B PNP | |1
C | |
| | |
+-----+-------+
|
GND

Unlike the vanilla 7555 which starts timing out when pin 2 goes low,
with the retrigger it starts timing out when pin 2 goes high. That
means that pin 3 will be high for the timeout period (1.4RC) plus as
long as pin 2 stays low:

A caveat: most jellybean PNP transistors have an emitter-base
breakdown voltage of around 7V, and are rated at around 5V absolute
max, so if you run the timer with a Vcc of = 5V it would be prudent
to put a diode in series with the emitter:


+V
|
+-------+
| |8
[Rt] +---+---+
| 2|_ Vcc |3
IN---+-------------------|--O|T OUT|-- OUT
| | 6| |
| +---|TH |
[1000] | 7|_ 7555|
| +--[1n4148]---+--O|D |
| | +| | GND |
| E [Ct] +---+---+
+--B PNP | |1
C | |
| | |
+--------------+-------+
|
GND






--
John Fields
Professional Circuit Designer