555 powered by a 2N3904?

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Doktor Jones

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I'm looking at running a 555 in astable mode, controlled by a 2N3904 or similar transistor; would it be possible to just run the supply voltage line through the 2N3904 (effectively using the transistor to turn the 555 "off" or "on")?
 
Doktor Jones said:
I'm looking at running a 555 in astable mode, controlled by a 2N3904 or similar transistor; would it be possible to just run the supply voltage line through the 2N3904 (effectively using the transistor to turn the 555 "off" or "on")?
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hi,
This one option.

As its a low power 555, I would use the RESET on the 555 rather than switch the Vsupply line.
 

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I'm with Eric. But using a transistor to switch the 555 supply would also be possible, providing the 555 load is within the transistor's current-handling spec.
 
hi ALL..
both of them are possible, but i think use RESET terminal give more advantage. if somehow 555 need to draw much current it won't overloading 2N3904.
 
RESET definitely seems to be a good idea... I'm just used to tying it high and forgetting about it! I'm trying to keep my design as compact as possible (it has to go into a very small compartment), so would this layout be a good way to simplify things a bit?

For reference, the 555 would just be driving an LED, so I don't think the load would be a concern for a 2N3904. One thing I thought of though... might switching the supply line result in a small (but potentially noticeable) degree of power savings? The device will be battery powered, and will probably only be receiving an "on" pulse for 2-4 seconds every few minutes.
 

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If need to control it with a 5V signal level, use a simple transistor, 10K from RESET to +V and the transistor collector to the other end of the 10K.

EDIT:

Low control , enables the 555
 

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An ordinary 555 produces a supply current surge of 400mA when its output switches. It is shown on the datasheet of the "better" ICM7555 that does not do it. An LMC555 and TLC555 are also Cmos without the high supply current surge. The 400mA might kill a little 2N3904. Use a 600mA-rated 2N4401 instead.
 
ericgibbs: I need the astable running when the input signal is high, so would it work to just switch Q2 and R4 (so RESET is pulled high when Q2 is on)?

audioguru: The chips I've got on hand are LM555CN; those are the CMOS versions, correct? Also, the reason I'm looking at 2N3904s is because I have them on hand...

Would switching the supply voltage result in any power savings (the device would be running off AA batteries in series), or is there no real difference?
 
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hi,
You cannot just swap over Q2 and R4. Tell me the voltage level you want to use, to control the RESET.

A LM555 is the old TTL version.

If you used a CMOS version the power consumption would be low. What do you plan to use the output of the 555, whats it driving..?
 
A Cmos 555 has a fairly low max output high current of 10mA when its supply voltage is about 10V or more. With a supply voltage of 3V its max output high current is about 1.2mA.
Its max output low current is 100mA when its supply voltage is about 6V or more. With a 3V supply its max output low current is about 6mA.
These are shown on graphs on the ICM7555 datasheet.

Cmos 555 ICs have a "C" in the part number. LMC555, TLC555 and ICM7555 are Cmos 555 timers.
 
So here's the circuit I'm working on. Basically, the input (Vin) will pulse high for ~1 second, triggering the first timer. After a user-specified delay (Vout1), Vout2 will go high for a user-specified amount of time; while Vout2 is high, the astable will output a series of pulses (Vout3) that will drive a single LED.

I've swapped C3 out for a 4.7µ cap for higher max frequency. R1 will actually be 50k multi-turn variable resistors and R2 and R4 will be 100k (all in series with 10k fixed); these look like they will give me the desired time ranges (~1s - 8s for Vout1, ~0.5s - 8s for Vout2, and ~1.7Hz - 14Hz for Vout3).
 

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hi,
It looks OK, some surplus components.? also add some +V decoupling.

Requires a base resistor in Q1
 

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