With reference to the schematic, the change in voltage at Vin of the LM393 lasts for around 10 seconds and therefore the relay switches for the same period.
Can anyone suggest an addition to the design so that the relay only switches for 1 to 2 seconds.
I cannot vary the 10 second input duration or level.
Many thanks
Tony

 

I would add a 555 timer in monostable configuration.

 

You can feed the relay transistor Base with a Series electrolytic capacitor, to time it out...

 

You can feed the relay transistor Base with a Series electrolytic capacitor, to time it out...

Yes, around 100uF should do the job but you should also add a reversed diode between the base and emitter of the transistor otherwise the capacitor won't be discharged.

 

I would add a 555 timer in monostable configuration.

Using this suggestion I would remove the relay from where it is along with D1 the flyback diode. I would place a 10K resistor between Q1 Collector and VCC. I would come off the Q1 Collector and drive the trigger input of a 555 Timer (pin 2) configured as a Non-Retriggerable OneShot Timer using a variable resistor to set the pulse width and have the 555 Timer drive your relay. A Google of 555 timer non-retriggerable one shot will yield dozens of circuits. Just remember the 555 timer can sink or source 200 mA so make sure your relay coil load does not exceed 200 mA.

or you can do as Doggy and Albert suggest. Simpler and less work and components.

Ron

 

Welcome to AAC!

Circuit in question (edited to remove some whitespace):


You can use the other comparator to make a one shot:

Time period determined by C1 and R4, R4 >> R3.

 

Yes, around 100uF should do the job but you should also add a reversed diode between the base and emitter of the transistor otherwise the capacitor won't be discharged.

Thanks everyone - I'll try these ideas to see which works best for me.

 

You can use the other half of the LM393 as a timer stage. Is the relay ON time always going to be shorter than the comparator output time?

ak