hello ,the 555 question , there are many ways of doing things,
a 555 can provide all the frequencys we want BUT , duty cycle or on time can only be varied between 50 % and 99% with a standard circuit , so on gary's PI and many others , the first 555 makes the frequency and the second is triggered to provide an output duration of aprox 160us on time (+output), it uses 2x555 to do this.
option 2 :
set the 555 to have 160us OFF time and use the zero volt output to drive a p-channel mosfet or pnp transistor,
this only needs 1X555
option 3:this is what i have done.
setup a 555 to do the frequency you require , then use a diode allowing current from pin 7 to the pin 2 and pin 6 join ,
this charges the capacitor fast , and the resistor discharges it slow , which restults in a short on time which can be varied with the pin 7 feed resistor to + whilst the pin 7 to 6/2 resistor varies the frequency.
the 555 chip is very versatile , and can be made to do anything from 5% duty to 95% duty.
the other 2x555's on garys PI are used in this way:
1st one gets triggered from the outut END of the TX timming output,waits for the delay period,
then triggers the 2nd 555 , which has a high output for the desired sample time.
all straight forward stuff , but there is no shortcut to using 2x555 for delay and sample,
but the 2x555's for TX pulse can be reduced to using only one by adding a 1n4148 or similar diode as mentioned above.
still , dam fine design , and using one more 555 than needed is neither here or there.
search for "blind squirrel" on the net , now that timming method i like simple , easy to play with , uncomplicated , when you get your head round it , only 2 chips used to do the lot TX ,delay and sample,and not many external components,
i like it.
Linkback: You are not allowed to view links.
Please Register or Login
http://www.thunting.com/smf/index.php/topic,12793.msg178104.html#msg178104
|
|
Logged
|
|