Output from bandpass filter to be amplified by transistor to trigger relay

[SimonFrisendal]

Hey.
I have a quick question about a project I'm building.
It can be seen in the attachment to this post, and my question concerns the bandpass filter section in the lower left corner. How do I determine the amperage coming from the operational amplifiers, so that i can properly calculate the resistor values in front of the transistors. If it should be needed, the relays need 18 mA to trigger.

Thanks a lot

- Simon.

 

[MrAl]

Hi,


You get that from the data sheet. Most likely it will put out at least 10ma but it's good to check first.

 

[audioguru]

Your circuit feeds the AC from the bandpass filters into the transistor that drives the relay then the relay will pulse on and off at the bandpass frequency. But you should rectify and filter the output of the bandpass filters to drive the transistors with pure and smooth DC when the signal is present.

 

[SimonFrisendal]

So like this? If the OPAMPs are supplied by -/+ 10 V, i can loosely account with 10 V from the op amps minus the 0.7 V across the diode. This plus the saturation amperage for the transistor can then be used to find the resistor value in front of the transistor, right?

 

[audioguru]

I think the rectifier diode plus filter capacitor will mess up the bandpass filter. You need a current-limiting resistor (about 470 ohms) between the output of the bandpass filter IC and the rectifier diode. Then calculate if it slows down the charging of the filter capacitor too much.

The output high of a TL082 opamp does not go closer than about 2V to its supply voltages.

 

[MrAl]

Hi,


Yes i agree that some rectification and DC filtering is a good idea, but one thing you have to watch out for is not to go too high on the filter capacitance or it could cause the relay to open too slowly when it is turning off. This could cause bad arcing between contacts depending on the application. So use a capacitor value that wont cause the relay to open too slowly, or add a parallel capacitor to help discharge the capacitor faster.

So this means a resistor in series with the op amp output, a diode such as 1N5817, a capacitor to ground with parallel resistor, then another resistor from the capacitor to the base of the transistor. A little trial and error should get you there or else we could Spice this thing. The coil itself holds energy for a short time too so that might help filter a little also.

 

[SimonFrisendal]

I think i will do a little trial and error and see if a the simple edition with a diode and small capacitor will work. If not i will try the other solution.