edge triggering a low power 555

[jwcornell]

Hi gang.

My first post so apologies if I mess it up.

I've got an application where I need to send a short pulse to a dc latching solenoid valve to open it and close it. The trigger will come from a vacuum gauge with an adjustable "trigger" that closes a circuit like an on/off switch at a user selectable vacuum level. The pulse needs to be only 6 - 12 volts for 10 milliseconds at 5 watts so not much power needed for that (I realize that I may need to go through a relay). The problem is that this device will be in areas where no power is available so it needs to be battery operated.

In reviewing various circuits it seems that a low power version of the 555 chip in monostable mode with edge triggering is the way to go but it seems to me that the edge trigger method (because of the resister needed) will likely consume too much power and there will be a need to replace the battery much more frequently. Does anyone have any ideas on how to trigger the 555 in another way using an on/off switch as described?

Since this switch needs to operate only once every two to five days and it will be outside, I was thinking that maybe using a very small solar cell to charge a capacitor which would be triggered when the switch is closed to trigger the 555 but I'm not sure how to design such a circuit. Any thoughts would be appreciated. Thanks.

 

[Boncuk]

Hi jwcornell,

welcome to the forum.

Your description appears somewhat vague to me.

Does the vacuum switch provide a steady signal (high or low)?

Do want to energize the load for a limited time (monostable) after the timer circuit has been triggered?

Regards

Boncuk

 

[jwcornell]

Answers to Boncuk

Hi Boncuk.

Thanks for the quick response.

The vacuum switch is essentially an on/off switch that opens and closes a circuit. It does not provide a signal. Most of the time, it would be in the open circuit state. When the vacuum reached a user selectable point, the switch would close the circuit. On average, the switch would remain closed for perhaps three to five hours before the vacuum receded and the switch went back to the open circuit state. It would not matter if the 555 triggered again when the switch went from closed to open as the solenoid it operated would already be open anyway.

The need is to have the switch trigger a pulse of 10 - 15 milliseconds between 6 - 12 volts and a load of about 5 watts.

It would only operate on average perhaps once every two to five days so this would be a monostable circuit.

I don't want to confuse anyone so I have limited this discussion to only the opening of the solenoid. Once this part is figured out, the same circuit design would be applied to closing the solenoid. The process of closing the solenoid is essentially the same as opening it but the polarity on the pulse is reversed.

Regards,

jwcornell