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| Single 555 Timer with 2 discrete duty cycles? (Not continuously variable) |
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| doublec4:
Hi all, I am using the typical 555 timer circuit to produce a PWM signal (see attached circuit). I would like two discrete duty cycles as determined by different R1,R2 combinations with a common C1. I would like to switch between the duty cycles electronically (no mechanical switches but rather with a digital I/O from microcontroller). Originally I thought about creating a type of SPDT circuit using complementary MOSFETs (between R1 and Vcc) to switch between different R1,R2 combinations in the circuit, but I don't think it will work as the different values for R2 would still be in parallel and it would mess up the duty cycle. Any ideas how I could achieve this switching between duty cycles? I have seen the circuits with continuously variable duty cycles using potentiometer but this is not what I am looking to do. Any help would be great! Thank you so much! |
| GerryR:
A single miniature DPDT relay (5V, 12V ??) would do it. No current to speak of and in excess of 10 6 cycles. Connect the junction of R1a and R2a to the NO of one set of contacts and the other side of R2a to the NO of the other set of contacts. Junction of R1b and R2b to the NC set of the contacts that the R1a and R2a set are connected to, with wiper to pin7 on the 555. Other side of R2b to the NC set that the other side of Ra is going to, with the wiper to the top of the Cap going to pins 2 and 6 of the 555. |
| doublec4:
--- Quote from: GerryR on June 26, 2019, 01:59:33 pm ---A single miniature DPDT relay (5V, 12V ??) would do it. No current to speak of and in excess of 10 6 cycles. Connect the junction of R1a and R2a to the NO of one set of contacts and the other side of R2a to the NO of the other set of contacts. Junction of R1b and R2b to the NC set of the contacts that the R1a and R2a set are connected to, with wiper to pin7 on the 555. Other side of R2b to the NC set that the other side of Ra is going to, with the wiper to the top of the Cap going to pins 2 and 6 of the 555. --- End quote --- Interesting, thanks! Makes sense and I can drive the coil with my I/O and transistor if the required voltage or current exceeds the capability of the I/O directly driving it. I tried looking up solid state DPDT relays on Digikey but it seems there are none. If anyone else has any other method of achieving this with mosfets or transistors or something I am definitely still interested in multiple solutions! Thank you |
| not1xor1:
--- Quote from: doublec4 on June 26, 2019, 02:42:29 pm --- --- Quote from: GerryR on June 26, 2019, 01:59:33 pm ---A single miniature DPDT relay (5V, 12V ??) would do it. No current to speak of and in excess of 10 6 cycles. Connect the junction of R1a and R2a to the NO of one set of contacts and the other side of R2a to the NO of the other set of contacts. Junction of R1b and R2b to the NC set of the contacts that the R1a and R2a set are connected to, with wiper to pin7 on the 555. Other side of R2b to the NC set that the other side of Ra is going to, with the wiper to the top of the Cap going to pins 2 and 6 of the 555. --- End quote --- Interesting, thanks! Makes sense and I can drive the coil with my I/O and transistor if the required voltage or current exceeds the capability of the I/O directly driving it. I tried looking up solid state DPDT relays on Digikey but it seems there are none. If anyone else has any other method of achieving this with mosfets or transistors or something I am definitely still interested in multiple solutions! Thank you --- End quote --- imho it would be easier to use a different circuit start from the 50% duty cycle schematic where the capacitor is charged/discharged by the output and then just change the CV pin voltage value to change the duty cycle accordingly for instance you can use a microcontroller I/O to connect CV pin via a 10k resistor to 5V or 0V or set the I/O open to leave the duty cycle at 50% |
| doublec4:
--- Quote from: not1xor1 on June 26, 2019, 04:23:39 pm --- --- Quote from: doublec4 on June 26, 2019, 02:42:29 pm --- --- Quote from: GerryR on June 26, 2019, 01:59:33 pm ---A single miniature DPDT relay (5V, 12V ??) would do it. No current to speak of and in excess of 10 6 cycles. Connect the junction of R1a and R2a to the NO of one set of contacts and the other side of R2a to the NO of the other set of contacts. Junction of R1b and R2b to the NC set of the contacts that the R1a and R2a set are connected to, with wiper to pin7 on the 555. Other side of R2b to the NC set that the other side of Ra is going to, with the wiper to the top of the Cap going to pins 2 and 6 of the 555. --- End quote --- Interesting, thanks! Makes sense and I can drive the coil with my I/O and transistor if the required voltage or current exceeds the capability of the I/O directly driving it. I tried looking up solid state DPDT relays on Digikey but it seems there are none. If anyone else has any other method of achieving this with mosfets or transistors or something I am definitely still interested in multiple solutions! Thank you --- End quote --- imho it would be easier to use a different circuit start from the 50% duty cycle schematic where the capacitor is charged/discharged by the output and then just change the CV pin voltage value to change the duty cycle accordingly for instance you can use a microcontroller I/O to connect CV pin via a 10k resistor to 5V or 0V or set the I/O open to leave the duty cycle at 50% --- End quote --- Sounds good... however, in my typical astable circuit, I was achieving less than a 50% duty cycle by putting a diode in parallel with R2. It got me down to about %20 by my calculation. Using the voltage controlled option on pin 5 I would still be limited to 50% as my lowest duty cycle, no? |
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