Trigger for 555Timer

Thread Starter

jboulasii

Joined Aug 16, 2009
11
I have a circuit in which i want the trigger for the 555 to be as soon as i apply power to the device. I was thinking of just taking the trigger directly yo ground. I want a one shot 5 second delay. With the trigger always grounded will I get my oneshot.
 

k7elp60

Joined Nov 4, 2008
562
To trigger the 555 on power up connect a resistor from pin 2 to Vcc. Connect a capacitor from pin 2 to ground. Upon power up pin 2 has a negative trigger.
 

Thread Starter

jboulasii

Joined Aug 16, 2009
11
Ive attached my full circuit. What im trying to do is change the bridge from full wave to 1/2 wave after some time peroid. I think that by turning off the two SCR's I should accomplish. I've isolated the gates using a pulse transformer with two secondaries. I'm hope is that on power up the 555 triggers and my count down starts say at 5 seconds. Once the 5 sconds is up I want the two SCR's to turn off.

Questions,

Do you think this conect will work?

Do I need the pulse or is there a better way?

Is i possible to use a straight RC newwork for the timming and not use the LM555? For me the fewer components the better.
 

Attachments

eblc1388

Joined Nov 28, 2008
1,542
A 555 is good, but it still needs a good power supply, not pulsating DC to power it up.
How true.

The common misconception is that 555 will keep its timing accuracy at whatever its supply voltage(within its rating) is.

This is only partly true.

The above statement should read: "The 555 will keep its timing accuracy at whatever STABLE supply voltage within its rating". This could mean voltage regulator for the supply voltage if one wants a stable timing output.

Any fluctuation in Vcc during/within a timing cycle will add inaccuracy to the timing.
 

Dragonblight

Joined Aug 19, 2009
35
RC network, OPAMP oscillator, or a standard two transistor osciilator.

Or you could just use a microcontroller so you can set up that tricky 5 second delay and play some NOP magic.
 
Top