If I get you, I think you want to enable/disable the 555 using the same
pushbutton, is that right?
If so, you can use a couple of inverters (cmos 4049 perhaps) along with a
normally open push button and and resistors/capacitors to do this.
.-----------------------------------------.
| |
| |\ ___ |\ ___ |
'----| >O-|___|--o----| >O----o---|___|---'
|/ 100k | |/ | 100k
| |
4049 | 4049 .-.
| | | 330k
| | |
| T '-'
| --- |
'-----o o----o
|
--- 1uF
---
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)
The pushbutton is normally open. The capacitor is charged to the output of
the right inverter. When you push the button, it changes the value of the
input of the right inverter, causing it to change its output. This is
independent of the starting state.
Use the real output (not the output after the 100k resistor) to drive the
reset of the 555.
Regards,
Bob Monsen
That looks great, but where do I put the trigger and reset lines from
the 555?
You set up the 555 as an astable multivibrator, and let it free run. You
connect this to the reset pin of the 555. When it is "high" output, the 555
will output pulses for you. When it is "low" output, the 555 will wait for a
"high" output, and output no pulses.
The typical setup for an astable is to have two resistors and a capacitor.
Connect the resistors in series with the capacitor between the voltage
supply and ground. Connect the discharge pin between the resistors, and both
the 'trigger' and 'threshold' to the cap:
-----------o-----------------.
Vcc(12v) | |
| .-.
| | | R1
| | |
| __ '-'
'---o| |o-----. |
.----o| |o-----)--o
output ----)----o| |o---. | |
reset ----)----o|__|o- | | .-.
| | | | | R2
| | | | |
| | | '-'
| | | |
'-------------o-)--o
| |
| --- C1
| ---
| |
GND | |
---------------------------o--'
1/f = discharge time through R2 +
charge time through R1 + R2
Discharge through R2 is .693 * R2 * C
Charge through R1 and R2 is .693 * (R1 + R2) * C
So, f = 1.44/(R1*C + 2*R2*C)
and d = 100 * R2 / (R1 + R2)
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
Regards,
Bob Monsen