Pulse Generator Schems?

[imix500]

Hi, anyone have a schem for a pulse generator that has a fixed On time of around 50ms and a max frequency of 10Hz? I basically need a PWM in reverse, adjustable off time. I am using a 555 in old reliable astable mode, but it doesn't like a duty cycle below 50%. Thanks!

 

[Sch3mat1c]

I've done it by connecting a PNP transistor as a CCS in place of "R1" (the resistor from +V to pin 7). This way it charges linearly (dv/dt = C * I... dv/dt = rate of voltage change) by the CCS and R2 and discharges just through R2 (the CCS' current is shunted through pin 7).

By varying base current to the CCS you can get an extreme duty cycle.

Tim

 

[imix500]

Thanks Tim, that a great idea. So, would the base be tied to pin 7 with the E tied to R2 and the C to V+? I'm having a little trouble visualizing it. How would the base current be varied? Thanks!
Ricky

 

[Sch3mat1c]

Collector is the high-Z output, aka CCS. Follow the current too, I think you described it reversed.. B to gnd through variable resistor (500k pot + 10k current limit R), C to pin 7 (7 to pins 6+2 thru "R2" as usual), E to +V.

Tim

 

[imix500]

Got it. Yeah, I did describe it reversed. It's a PNP device, I had the current flow backwards. This is why I don't design after a few pints. Thanks!
Ricky

 

[Pjotr]

Hi Ricky,

Another way to do it is use a resistor in series with a diode (i.e. a 1N4148) from the output to the cap to charge the cap, this sets the fixed on time. Use another (variable) resistor from the discharge pin to the cap to discharge the cap and set the off time. This works best with CMOS 555’s.

Cheers

 

[imix500]

Pjotr, do you happen to have a schem using that arrangement? Thanks!
Ricky

 

[imix500]

Pjotr, just found a schem using exactly what you described. I will build both and see what works best. Thanks for all the help!
-Ricky

 

[wrenchone]

The classic 555 astable circuit can be fixed to supply <50% duty cycle by shunting the discharge resistor with a diode. This is probably the simplest way.