Electronics > Projects, Designs, and Technical Stuff
Need some help with a 555 timer circuit
Pizzashape23:
In terms of changing the transistor, if I was to swap it for a BC337 with a beta(min) of 60, assuming a Max current of 300ma (150ma per relay) could I get away with using the same 1k resistor, or should I use something closer to 500ohms?
Ian.M:
If you are using a bipolar 555 e.g the original NE555, its output high level is up to 3.25V below Vcc. Assuming 12V -5%, that means worst case it could be only 9.15V. Typically it would be somewhat better but you cant rely on typical values. My 1K base resistor and pulldown divide that by two but drop the source impedance to 500 ohms. Assuming Vbe of 0.7V, that gives you 7.7mA base drive, which will get it into saturation at 300mA Ic but with only 50% margin. I'd be happier with a bit more base drive. Decreasing the base resistor to 470R while keeping the pulldown at 1K increases the base current to 17.3mA, which is more than adequate base drive.
If you are using a modern CMOS 555 its output will get somewhat closer to Vcc even when sourcing 10mA, so the base drive will improve a bit, but not enough to significantly affect the design.
eblc1388:
--- Quote from: Ian.M on August 12, 2020, 10:16:36 pm ---You are probably right, and that would certainly help with EMI immunity, but I'd like to see a fully worked up design of the single 555 approach.
--- End quote ---
I tend to go along with Zero999. Here is my feeble attempt to create what the OP wants. It survives the contact bounces at release. 555 Trig pin is pulled up via a 2K2 resistor which is fairly strong and should prevent normal EMI false triggerings. I just pull the component values from thin air and perhape you can fine tune it better.
Pizzashape23:
This is very similar to my original shcematic posted on StackExchange, however many people pointed out that getting rid of the resistor across the cap and pulling up the switch using a 10k resistor would be better, and for the sake of simplicity the 2k2 resistor could also be 10k. At the moment I am leaning much more towards the dual 555 design, only because i am already printing the pcb so the added complexity and few extra components do not impact time or cost significantly, and because it is supposedly better protected and more stable. In my opinion it's a no brainer, although there is no doubt that it could be simplified.
Ian.M:
Eblc1388's implementation of Zero999's circuit does quite well, and is a considerable simplification compared to mine. With a couple of tweaks it would be entirely acceptable, provided high level EMI pickup on the button wiring is avoided (e.g in hostile EMI environments, use twisted pair and keep the wiring as short as is reasonably practical).
IMHO the 2K2 pullup should be reduced to 1K to get enough wetting current, a 1K (or lower) resistor should be inserted in series with the Discharge pin to control the current, the 100K resistor across the input cap should be made the same as the 91K timing resistor for BOM minimisation, and the 100nF caps should be increased to 220nF to increase the button up debounce period. Also the supply decoupling should be 10uF if a bipolar 555 is used.
N.B Zero999 failed to spot the other topic was using 5V levels. The power supply SPICE line needs to be changed to:
--- Code: ---Vvcc Vcc 0 12V ; power supply
--- End code ---
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