Trying to understand this 555 circuit I used a long time ago... - Page 1
Forum section: electronics / beginners

author: doublec4
author_rank: Regular Contributor
timestamp: April 09, 2021, 04:21:44 am
content: So a few years ago I put together this 555 timer circuit that I was using to flash some LEDs. In my eagle schematic it says it flashes about once per second. See attached eagle schematic. I cannot seem to find any other documentation I saved as to why it works... but I still have the PCBs and they work.Now looking at some of the typical astable circuits online and in the datasheet there are some clear differences.Hoping someone can help me understand my old circuit as I can't seem to piece it together in my head anymore.Thank you,EDIT: Just to add to this thread... my goal was re-use the circuit to drive the gate of an N channel mosfet so I could control higher power LEDs. The flash rate is suitable for what I want but I'd like to understand how/why this is different than the typical 555 led flasher circuit.

author: fourfathom
author_rank: Super Contributor
timestamp: April 09, 2021, 04:38:52 am
content: Rather than using the typical configuration using the Discharge pin, the 555 can also be made to oscillate by feeding the output back to the Threshold pin via a resistor. I don't recall the details (it's been a *long* time since I played with a 555), but I think it gave you better control of the duty-cycle.

author: doublec4
author_rank: Regular Contributor
timestamp: April 10, 2021, 06:43:46 am
content: Quote from: fourfathom on April 09, 2021, 04:38:52 amThat does sound familiar. I believe with the typical arrangement I couldn't get the flash frequency that I wanted... I just can't recall how or why this works how it does.

author: doublec4
author_rank: Regular Contributor
timestamp: April 10, 2021, 06:44:28 am
content: Quote from: camila on April 10, 2021, 05:04:56 amNope, the flash frequency is fine with this circuit, I just could not remember how or why it worked...

author: pqass
author_rank: Super Contributor
timestamp: April 10, 2021, 06:55:55 am
content: Quote from: doublec4 on April 09, 2021, 04:21:44 amThe 555 output can both source and sink current. By using the output instead of the discharge pin, C2 is both charged and discharged symmetrically through the same R7. R6 doesn't contribute much so it's probably used to trim R7 since it's almost 10 times the latter and won't change the timing much. However, it does make the off-time a little longer (see attached).It is similar to how inverter oscillators work. See:https://www.allaboutcircuits.com/textbook/experiments/chpt-8/555-hysteretic-oscillator/The typical 555 oscillator will have a different charge vs. discharge timing since the discharge pin will ground the node at 555 pin 7 for half the cycle which cuts the combined R1+R2 resistance (in second image).See attached falstad.com plots; with R6, and without R6 connected.Play with it here:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0mQrOaAmMAOZAWAnAZmwGyQDskumxuI8k11t8ApgLRhgBQYBi8xBIySGhAE0wwcP4w4kLtkEFKhTBjAU07AO4hSmAZlq7wfKOwDmI4shC5Rl65i5RT2ggeP8jYE5HYAlHUg9b08g8DBrWlp3COco6HgtahNcW2T+XDDfAGN0mzTefgMoqFgZZGxKquqa4hFYbC42NHISeFxhaVlzPOLepyj2XLdaW0l3PtowFmFOso4LEZA+pccpUwAnPMy9TFxrEOdkQRkymXOyJL2DkyXD320jZCtA3f2XV+X366-I9i2fj9CjYsjYiHB2AAHT7PaxoJyw8BxHrw-iItAqAQvQbQ4E7X4g4LIiw-fFrQnIgD24EQEGitmsglKdKRAHkAKoAFQAClyADoAZ25AEkAHKC3DsaneJH0ggHeIsiCcvwAGk5AAk-IKRaKAMqC5CqgjsIA

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2021, 02:38:19 pm
content: Quote from: doublec4 on April 09, 2021, 04:21:44 amNote that if you used the bipolar NE555, it probably won't flash at the same frequency and duty cycle, predicted by the simulator, which will assume the output stage swings over the full supply voltage range. In reality, the 555 has an asymmetrical output stage, with slightly more voltage drop on the positive, than the negative side, which will cause the capacitor to charge more slowly, than discharge.Awhile ago, I made a detailed LTSpice model of the 555 timer's output stage. Here it is connected as a Schmitt trigger. It should have a duty cycle of 50%, but it's higher than that, because the output stage saturates at 1V to 1.4V below the supply, depending on the current, when its positive.