Pulse Generator Schems?

imix500 · 8th April 2004, 04:23 AM

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 · 8th April 2004, 05:00 AM

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 · 8th April 2004, 05:47 AM

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 · 8th April 2004, 04:22 PM

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 · 8th April 2004, 06:13 PM

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 · 8th April 2004, 08:17 PM

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 · 8th April 2004, 08:52 PM

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

imix500 · 8th April 2004, 09:07 PM

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 · 9th April 2004, 02:55 AM

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.

8th April 2004, 04:23 AM	#1
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Pulse Generator Schems? 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!

8th April 2004, 05:00 AM	#2
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	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 __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

8th April 2004, 04:22 PM	#4
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	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 __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

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8th April 2004, 05:47 AM	#3
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	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

8th April 2004, 06:13 PM	#5
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	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

8th April 2004, 08:17 PM	#6
Pjotr diyAudio Member Join Date: Nov 2002 Location: Netherlands	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

8th April 2004, 08:52 PM	#7
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Pjotr, do you happen to have a schem using that arrangement? Thanks! Ricky

8th April 2004, 09:07 PM	#8
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	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

9th April 2004, 02:55 AM	#9
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	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.

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