555 burst timer - Page 1
Forum section: electronics / projects

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 27, 2021, 03:01:55 pm
content: I'm trying to design a burst generator with two 555 timers. The first timer will generate a 40khz square wave, this is working fine. I was planning to use a second 555 to control the control voltage of the first 555. So i can have a perdiod (say 10ms) with 40khz square wave as an output. The other half of the period the signal should be zero. For some reason it looks like my CV is oscillating. The only solution i could come up with was a 1 ohm resistor between the control voltage and ground. This doesn't looks good to me. I cant figure out why the CV is oscillating. Any suggestions?

author: Jan Audio
author_rank: Frequent Contributor
timestamp: October 27, 2021, 03:58:18 pm
content: You should power 1 555 from the other 555 output.Its possible, i saw it somewhere, if that is what you mean.

author: StillTrying
author_rank: Super Contributor
timestamp: October 27, 2021, 03:59:55 pm
content: Apply the 10 ms to the 40 kHz's RST pin?

author: Raj
author_rank: Frequent Contributor
timestamp: October 27, 2021, 04:45:12 pm
content: or simply use microcontroller :pon attiny13a running at 4.8Mhz, I forgot how to do it exactly but you can run a timer pwm for signal and do the 10ms delay in software(using Arduino or microchip studio library).Do tell me if you need the code and how you want it to operate.

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 27, 2021, 04:54:35 pm
content: Quote from: StillTrying on October 27, 2021, 03:59:55 pmYES! this is the solution! Thank you for the feedback!I know there are other ways to achieve this ( i would also have used a microcontroller ). For this project i had no other choice than a 555....

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2021, 05:19:34 pm
content: Quote from: Jan Audio on October 27, 2021, 03:58:18 pmThat will work, with two separate 555s, but the original poster might want to use the 556 for the real thing.Quote from: StillTrying on October 27, 2021, 03:59:55 pmYes. That is the best way to enable/disable an astable 555 timer circuit.Quote from: reyntjensm on October 27, 2021, 03:01:55 pmThat will not work.Altering the control voltage pin affects the duty cycle and frequency of the astable. It doesn't just enable/disable it. It's good if you want the output to switch, or sweep between two frequencies. I made a nice sounding alarm with two 555 timers for a school project, many years ago.Use the reset pin to enable/disable the astable, as mentioned above.Note that, unless you're using the CMOS 555, your circuit will not produce a 50% 40kHz duty cycle square wave, as shown by the simulator. In reality, it will have a slightly lower frequency and higher duty cycle, as the 555 time model included with LTSpice doesn't accurately simulate the output stage. It assumes it can swing all the way to both power supplies, when in reality, it falls short of the positive supply by around 1.2V. If you're using the LM/NE555 then a low value pull-up to the discharge pin, will give better results than connecting the timing resistor to the output pin.Another thing is there's no point in using precision resistor values such as 7412 and 1786, because the tolerance of the capacitors will be 5% or worse.Here's a revised schematic, with practical component values and drawn in an easy to read manner. You'll need to add a 1µF capacitor across the power rails, in the real circuit, to provide supply decoupling. 100nF is not enough for the 555 timer.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 27, 2021, 05:33:17 pm
content: The reset pin can be driven by the LF oscillator- that should work but you may find similar probelms if its power related.If this is caused by Vcc rail noise feeding through the CV input, try a 10R resistor with decoupling on both sides to separate the two Vcc's. Or try 7555s. You could let both 555s free run and feed the two outputs into an and or nand gate. You can use mickey mouse logic if you like:You can build your gated oscillator from CMOS logic eg 4011. A CD4093 will work the same way but with less drift (but with a bit more supply current)and if you want some cool latching action see this brilliant use of XOR CD4070.If you need precise 50% DS, then a T type FF can to be driven from 2f on both low and high frequency oscs.

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2021, 10:34:17 pm
content: Here's a 555 timer circuit which gives 40kHz, with a duty cycle very close to 50%, using a bog standard NE555. It uses less power than the other circuits, as there isn't low value pull-up between the discharge and +V.For about 100Hz, use R1 = 51k, R2 = 120k & C1 = 56nF.The reset pin can be used to enable/disable it, as per my previous schematic.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: October 28, 2021, 03:33:01 pm
content: 556 is asking for trouble.2 x 555 is better, then they each have a seperate 100n and no problems.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2021, 04:45:14 pm
content: Quote from: Jan Audio on October 28, 2021, 03:33:01 pmWhy is that an issue? Just use a larger capacitor. Note the plain old bipolar 555 timers need a larger decoupling capacitor than 100nF. I'd recommend 1µF minimum.The only reason I can think of for using two separate 555 timers, rather than the 556, is two devices can dissipate more power, so can be both used to the full output current of 200mA. The only other reason to use two 555s is if that's all you have available.