Problem with timer part of circuit

[angie1199]

Hi guys

I've got a circuit that works to a degree but I needed to make a modification. Below are links to the 2 circuits.

The first works in as much as it has an instant on-off action when current is sensed across the bridge (8a-8b). The circuit, given a stable connection, will switch transistor T7 and switch the red LED on, and green off. This is instant (milli-seconds) and would, in ideal circumstances be fine. Unfortunately the source of the current across 8a-8b is not as continuos as I would like and cannot be made firmer, so the intermittent switching oscillates the red and green LED's.

Circuit 1
**broken link removed**

So with that in mind I hunted for a time delay circuit and found the 'timer' circuit as shown in Circuit 2 below. I had hoped that putting it between the 'detector' and the 'signal control' circuits would work straight away as now, when current flows through 8a-8b it would trigger the 5 second timer which would, in turn, activate the red signal. Then any short breaks in the current across 8a-8b would be covered by the 5 second delay, and the 5 seconds would be restarted.

Circuit 2
**broken link removed**

Unfortunately with the 'Timer' circuit added, the timer is triggered immediately the 'Detector' circuit is connected, even without a current across 8a-8b. This switches the signal to red. Once the time delay is up, the signal switches to green for less than a second and then the timer is activated again.

I have no idea what is triggering the timer as nothing should be switching transistor T10, and even if it was, why then would the timer expire and restart when this circuit should reset while the timer is active if triggered again, so should never run out.

The switch SW1 is a bypass and when the detector is not connected it activates the timer as it should so I know the 'Timer' circuit with 'Signal Control' works.

Anyone any ideas what may be/is triggering the timer in 'Circuit 2' or maybe another way to add a release delay to 'Circuit 1' without using the timer circuit in 'Circuit 2'?

Hope this makes sense and sorry for the large images but couldn't read the labels when I down-sized.

Angie

 

[ericgibbs]

hi angie,
I quick look at the 555 timer shows that the RESET pin is not connected.??
Depending upon the circuits overall time constants, power supply etc, you will find that the 555 will trigger as its powered up.
You could add and Res/Cap RESET to to the 555 RESET pin.

 

[angie1199]

Eric,

Thanks for taking a look.

The 12V power supply is DC and the current across 8a-8b is 13-16V square wave 12-16khz.

Triggering on power up isn't really a problem if it then went off until triggered by current across 8a-8b, but it keeps triggering every time the 5 second timer expires. It doesn't trigger when powered up if the detecotr part is not connected, seems like the detector part is triggering it but it's not supposed to unless current goes 8a-8b.

Apart from a basic knowledge of what each component does I have little understanding of how they integrate. The datasheet shows the reset pin is 0.7V typ./1V max. The reset current is shown as +0.4V/0.1mA typ and +0.4V/0.4mA max. With the powersupply being 12V, do I multiply 'up' so to speak to get the correct resistor value? And how do I select a capacitor, is it just a smoothing type so 100nF would be sufficient?

Regards

Angie

 

[ericgibbs]

Eric,

Thanks for taking a look.

The 12V power supply is DC and the current across 8a-8b is 13-16V square wave 12-16khz.

Triggering on power up isn't really a problem if it then went off until triggered by current across 8a-8b, but it keeps triggering every time the 5 second timer expires. It doesn't trigger when powered up if the detecotr part is not connected, seems like the detector part is triggering it but it's not supposed to unless current goes 8a-8b.

Apart from a basic knowledge of what each component does I have little understanding of how they integrate. The datasheet shows the reset pin is 0.7V typ./1V max. The reset current is shown as +0.4V/0.1mA typ and +0.4V/0.4mA max. With the powersupply being 12V, do I multiply 'up' so to speak to get the correct resistor value? And how do I select a capacitor, is it just a smoothing type so 100nF would be sufficient?

Regards

Angie

Angie,
I will take a closer look at the second circuit, get back to you.
If the RESET on the 555 is not used, its usually connected directly to the +V12, never leave the pin floating.
IF you did require a power RESET [future work maybe] , connect the RESET pin to +V12 via a 47K and 1uF capacitor from the RESET pin to 0V.

 

[ericgibbs]

hi Angie,
The problem is that 'vc', refer the LTS sim image, is always high, even with no 15Khz signal. Which is enough to keep the trigger collector low , 0v, so the 555 will keep re-triggering.
Consider a coupling capacitor to the trigger input, to block the low level.

 

[angie1199]

Thanks Eric

Ok, so I added a 300nF capacitor from trigger to ground and it reduced the timer time so increased the timer capacitor to 470uF and it's back to 5 seconds. The circuit works ok in TINA so now to try it out on the breadboard. I have the detector and signal control built, just need to build the timer section on the breadboard.

Will let you know how it all works out

Angie

Edit: Ah, forgot, I also connected Reset to Vcc

 

[angie1199]

Ok, I'm officially an idiot

To etch my pcb I drew the tracks and holes in paint shop pro. I put the components on a second layer so I can print it off to see where the components go when building. I had transistor T10 in Circuit 2 round the wrong way!

Thanks for your time Eric, with this numpty. It works fine now. I've left the additional capacitor in there although the only one I had was for a little more power than required. I'll get some smaller sized ones for the build. I only have about 30 of these circuits to make up.

Best regards

Angie