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| 555 astable timer help |
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| m3vuv:
hi all,ive built an astable 555 timer with the intention of using the output pulses to switch a mosfet to drive a solenoid vave,thing is i didnt realise pin 3 output is a cuppent sink not a source,its opposit to what i expected,any idears of a way to overcome this?,its got me stumped,ps. the power budget is tight as its battery poweredi ie whatever i use must have a low quiessent current.,cheer Paul m3vuv. |
| jhpadjustable:
--- Quote from: m3vuv on September 05, 2019, 11:44:58 pm ---whatever i use must have a low quiessent current --- End quote --- Low Iq is not the 555's specialty. An MCU would be much more effective. Pin 3 is push-pull. If your problem is that the sense of the output is inverted (active-low) from the sense that you want (active high), change your want and turn the MOSFET and solenoid segment upside down instead. Note the change to p-channel MOSFET, other circuitry and protection diodes not shown for clarity, drawing not to scale, etc. etc. |
| Zero999:
--- Quote from: jhpadjustable on September 06, 2019, 12:10:33 am ---An MCU would be much more effective. --- End quote --- How do you know that? There isn't enough information to determine whether a MCU would be a better choice, in this application. There are plenty of reasons to use a 555 timer, over a microcontroller. For example, if it's running off 12V, a 5V regulator and a logic level MOSFET would be required with an MCU, but not with the old 555 timer. --- Quote from: m3vuv on September 05, 2019, 11:44:58 pm ---hi all,ive built an astable 555 timer with the intention of using the output pulses to switch a mosfet to drive a solenoid vave,thing is i didnt realise pin 3 output is a cuppent sink not a source,its opposit to what i expected,any idears of a way to overcome this?,its got me stumped,ps. the power budget is tight as its battery poweredi ie whatever i use must have a low quiessent current.,cheer Paul m3vuv. --- End quote --- A 555 timer can both sink, as well as source current. In fact it's capable of sinking/sourcing more current, than most microcontrollers, so I don't think that's the issue here. Please post a schematic. |
| Renate:
Ah, same project, different thread. A 555 with a standard two resistor astable can only deliver an output which is more of the time high than low. Pin 3 (totem pole output) and pin 7 (open collector) operate in the same direction all the time. Want independently adjustable on and off times? Use the pin 3 output through two steering diodes and two pots to the C/2/6. The P MOSFET is a good idea, as long as the solenoid supply is the same as the 555 supply. Of course, as a joke you could drive the P MOSFET with pin 7 and a pullup resistor. I'll bet you could run a lot higher voltage than the absolute maximum (18V) for the whole chip. You could power the 555 with 5V and maybe run 48V on pin7. Who wants to try? Of course a solenoid valve won't give you much accuracy on dispensing. I presume that you've already ruled out a peristaltic pump? |
| Zero999:
I don't know what the maximum voltage rating of the discharge transistor (pin 7) is. I doubt it's much more than the maximum rating of the IC. To use it at higher voltages, add another NPN transistor (base to +V, via a suitable resistor and emitter to pin 7) to form a cascode with the internal discharge transistor. |
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