Relay doesn't cut off.

snug_gles said:

33uF tantalum or also using 22uF tantalum.

Want to get at least 2 minutes from the 555 timer.

Will test with a lower value resistor to increase the current to the cap later.

Click to expand...

I am just wondering if the leakage current thru the 33uF cap is about the same current thats charging it via the 5.6M.?

I know that by trying a lower value resistor you will have a shorter delay, but its just a test set up.

If it works OK, you could try a 100uF tant, with a 5.6/3 = 1.8M

snig_gles,

Remove the 1N4004 from the base of the transistor. When the 555 output goes low, the diode is effectively reverse biased and the base of the transistor is floating. Any static or electromagnetic fields can partially turn on the transistor...and your case prevent it from turning completely off.

Ken

Did you note this comment of mine?

Roff said:

If your vibration switch is still open when the 555 times out, the output will stay high until the switch closes. If this is the case, you need a differentiator between the switch and pin 2, so the input pulse is shorter than the expected output pulse. A differentiator for this circuit needs special attention, or it can destroy the 555 with overvoltage on pin 2 when the switch closes.

Click to expand...

Or this one?

Roff said:

I think you may have hit the nail on the head. Unfortunately, if you make C smaller, R has to be larger, and you may have the same problem. One way to guarantee that the circuit will time out (although it will be shorter than predicted) is to put the timing capacitor in parallel with the timing resistor. The leakage now aids the resistor, instead of fighting it.

Click to expand...

I'm not saying either one is the solution to your problem, but I don't believe you commented on either one, so I thought you might have (dis)missed them.

use something like a 4017 counter and clock it with the 555 at a slower spedd 1/10, so instead of getting a pulse every 210 seconds, a pluse every 21 seconds, use 2 4017s and it becomes 2.1 seconds. All this adds to the complexity and board size increases

Hi Roff,

Thanks for the reminder. I am looking at those suggestions right now.

Will update when as I get more soldering and checking done.

Cheers.
------------------
Hi Supervoip,

Trying to keep it as small and simple as possible.

Hi Roff,

You're right about the motion switch retriggering the 555 timer once it times out.

I replaced the motion switch with a jumper contact and the 555 timer and relays operate exactly as expected.

Problem I have now is how to use the normally closed motion switch (maybe too sensitive) in the circuit.

I was a bit lost with the op-amp for the differentiator and how I could construct the circuit.

My thoughts now are to use a 556 timer and then have a delayed re-trigger between the two timers, so that when the first 555 timer times out, it can not be retriggered until the second 555 timer times out. This way, I can be sure of the whole thing switching off. Then when the second 555 times out, then if the switch is triggered again, the whole count can restart.

Your thoughts please.

Are you saying the sensor is still switching after 3.5 mins? Can you point to a datasheet?
I wasn't thinking of a 'toggling" sensor. Is that what you have?
I guess I was thinking you were using something like a tilt switch, and that it might still be "tilted" at the end of the planned timeout, which would force the 555 to still be on.

Hi Roff,
I'm using an Assemtech MS24M motion/vibration sensor.
Hmm... just re-read the datasheet for the sensor, it has not particular state... n/c or n/o.. detects change of state from n/o to n/c or vice versa... this presents me with another problem.. doh.
When I use this sensor, some times the relay cuts out, but sometimes it doesn't.
I think that the sensor is causing the 555 to retrigger.

snug_gles said:

Hi Roff,
I'm using an Assemtech MS24M motion/vibration sensor.
Hmm... just re-read the datasheet for the sensor, it has not particular state... n/c or n/o.. detects change of state from n/o to n/c or vice versa... this presents me with another problem.. doh.
When I use this sensor, some times the relay cuts out, but sometimes it doesn't.
I think that the sensor is causing the 555 to retrigger.

Click to expand...

A microcontroller solution would have fewer parts, and solve your long timeout problem. If you still want to use a 555, you can use an Exclusive OR gate to give you a negative pulse on any transition of the switch. See below.
If the circuit is too sensitive, the MS24M datasheet shows how to solve that problem.

Your schematic will be cleaner if you save it as a GIF or PNG instead of JPG.

Hi Roff,

Got all the bits for the circuit that you proposed, wired it all up.... now the sensor is not sensitive at all... I can shake it around for awhile before the XOR circuit triggers the 555.

Without the XOR circuit, the slightest push will trigger the 555.
With the XOR circuit, I can shake the sensor vigorously, and only after a while, it will trigger the 555.

I can't figure out the first XOR gate, with the sensor and the 100k resistor, how that would work. If the sensor triggers, but both inputs are jumpered, then how does the XOR gate differentiate between 1 and 0 of both inputs?

Thanks... learning heaps.

snug_gles said:

Hi Roff,

Got all the bits for the circuit that you proposed, wired it all up.... now the sensor is not sensitive at all... I can shake it around for awhile before the XOR circuit triggers the 555.

Without the XOR circuit, the slightest push will trigger the 555.
With the XOR circuit, I can shake the sensor vigorously, and only after a while, it will trigger the 555.

I can't figure out the first XOR gate, with the sensor and the 100k resistor, how that would work. If the sensor triggers, but both inputs are jumpered, then how does the XOR gate differentiate between 1 and 0 of both inputs?

Thanks... learning heaps.

Click to expand...

You are correct. Brain fart.
I have edited the schematic.