sa555 problem

Hy, first I sorry for my english

I want to make a simple mono stable 555 timer circuit. I use the sa555 because is guaranteed high temperature range (-20 to +85). I was make the circuit its work to 25 seconds, but if I setup 30 seconds ore more that don't want to turn off (tested in -10 C, in normal conditions - room temperature all its ok but if go out in -10 -15 don't work).
I want to make a feeder.
Components what I use:

SA555 timer
relay
10k resistor
100uF capacitor
47uF capacitor
2×10nF capacitor
500KR for 1-60 seconds

Thanks for response

I don't now where is the problem all components what I use is in the range -20C+85C

Please post a schematic. I'm guessing that capacitor leakage is your problem, but I cant tell which capacitor does what in your circuit?

my schematic

MikeMl said:

Please post a schematic. I'm guessing that capacitor leakage is your problem, but I cant tell which capacitor does what in your circuit?

Click to expand...

here is my schematics

That is a printed circuit layout. Don't you have a schematic diagram of the circuit?

circuit layout

Lori:

If I were grading your circuit in my class, I would give you a C-.

1. D2-Q2 base circuit. You need a resistor to limit base current to a safe value. Depending on what the input voltage is, it could be ~ 1K...

2. RL2 is not even needed. You could trigger the 555 delay cycle without it. If you intend to keep it, you should have a snubber diode (1N400x, cathode to +12V) around it's coil.

3. C4 is too large. It will burn and pit RL2's contact unless you put a current-limiting resistor in series with the contacts. See 2 above. Another reason for getting rid of RL2.

4. R1/C3; the time constant is too short. This may be why your timer is unreliable (i.e. it fails to trigger sometimes).

5. VR1 should have a resistor in series with it to limit the peak inrush current. If you run the pot wiper near minimum resistance, you will likely blow up the pot.

6. D1/RL1. Diode should be 1N400x (1<=X<=7). No good reason for D1 to be in-series between pin 3 and the relay. Hook the top of the relay direct to pin3, and use the diode as a snubber; cathode to pin 3 and anode to gnd.

7. Probably need a snubber diode around the motor, too...

btw- my consulting rate is $250 per hour

Thats a pretty damn good answer right there. (Not brown-nosing mike, but you do seem to cover answers fully )

lorri, if you follow those steps and ask yourself why Mike asked those questions, you will have a much better understanding of the circuit, and what each component does, and why it is there. After those possible modifications, I see no reason why it wouldn't work at -10C.

C4 is much too large and will take a long time to discharge when power is removed. I don't see how it would ever actually discharge fully, unless maybe it powers the 555 long enough to discharge through VR1 and pin 7. The circuit would fail to trigger if the power is only removed for more than the VR1/C4 time constant. If the circuit is set for 30 seconds, then it will not trigger until after power has been removed for a similar length of time.

The circuit has been discussed and graded already.

Here is a simple circuit, adjustable from 1.5 to ~34 seconds (no other value, e.g. 450K available).

To trigger the circuit pull pin2 of the timer to ground which starts timing.

The screenshot shows the circuit active (LED on and relay pulled in).

For the timing cap try to get one with low ESR and low tolerance. (If necessary salvage one of your PC mainboard. Must be rated 16V though.)

Values other than 47µF make timing difficult.

You should consider to use a TLC7555 (CMOS version of the NE555), which won't require large value timing caps for 30 seconds, and use a high capacitance polypropylene type cap, such as WIMA MKS-2 with a tolerance of 5%.

Electrolytic caps might have a tolerance of ±40%.

Boncuk

mneary said:

C4 is much too large and will take a long time to discharge when power is removed. I don't see how it would ever actually discharge fully, unless maybe it powers the 555 long enough to discharge through VR1 and pin 7.

Click to expand...

It discharges through the supply pins of the 555.

Boncuk

lorri said:

I was make the circuit its work to 25 seconds, but if I setup 30 seconds ore more that don't want to turn off (tested in -10 C, in normal conditions - room temperature all its ok but if go out in -10 -15 don't work).
I don't now where is the problem all components what I use is in the range -20C+85C

Click to expand...

What type of cap is C1? (Your 100uF timing cap) Note that electrolytic caps have relatively high leakage, which slowly decays, when power is applied to them. Also their capacitance drops off a lot below freezing while their ESR also rises.
A tantalum cap would have much better temperature stability and lower leakage than an electrolytic.