Hi everyone, I am currently doing a mini project about piezocantilever that power up the RF. In order to power up the rf, a capacitor is needed. Right now I discharge my capacitor to power up transmitter by pushing a button. Once it is discharged, there will be no power to my transmitter then I released the button to let the capacitor to charge again. So is there any module or circuit which can control switch on/off by detecting voltage drop/increase automatically?

 

So you want your transmitter to power on when the capacitor’s voltage reaches a threshold, and off when is decays to a threshold, without constant power to detect it?

 

Yes, there will no no constant power source on the transmitter side. I heard of something that can detect the voltage threshold and gives out signal to a connected transistor that act as a switch. Voltage monitor module?

 

To what voltage is the capacitor charged? Is the charger turned off during discharge, or is it on continuously?

 

To what voltage is the capacitor charged? Is the charger turned off during discharge, or is it on continuously?

The capacitor is charged by a regulated 3.3V from ltc3588. The charger is turned on continuously during discharge

 

At that low a voltage, you can use a comparator, assuming you have a voltage reference, to give you an electronic switch.

 

At that low a voltage, you can use a comparator, assuming you have a voltage reference, to give you an electronic switch.

The trouble is there is no power in the off state. But perhaps charging a small cap on the device side could power a comparator circuit.

 

The trouble is there is no power in the off state. But perhaps charging a small cap on the device side could power a comparator circuit.

I think that condition is still subject to translation.

Yes, there will no no constant power source on the transmitter side.

That doesn't tell me there is no poser on the charging side.

 

I think that condition is still subject to translation.

That doesn't tell me there is no poser on the charging side.

It’s a piezo button.

 

It’s a piezo button.

But the charger has power continuously:

The charger is turned on continuously during discharge

I was thinking of a comparator controlling an SCR or mosfet switch. The continuous charging maybe a problem for the former, unless a GTO version is used. There is still a window of ambiguity, and a schematic would certainly help.

 

But the charger has power continuously:


I was thinking of a comparator controlling an SCR or mosfet switch. The continuous charging maybe a problem for the former, unless a GTO version is used. There is still a window of ambiguity, and a schematic would certainly help.

I may misunderstand this. It is my impression that he is using a kineticaly powered swtich which is being used to power a small transmitter when it is pressed, This is a typical IoT application, so maybe I am just imposing my expectations on it.

Schematic desired.

 

 

OK, so do you want to enable/disable the transmitter or turn on/off the power supply output?

 

Since the power supply output is the sole power of the transmitter, won't turning on/off the power supply output = enable/disable transmitter?

 

Since the power supply output is the sole power of the transmitter, won't turning on/off the power supply output = enable/disable transmitter?

It will, but doesn’t the supply output naturally decay to the point of not operating when the voltage drops?

 

Let me back up a minute. Am I right in understanding that you want the transmitter to stop drawing current once the capacitor is discharged to allow it to recharge, then transmit again if it does?

 

The LTC3588 is an interesting chip. Since with a capacitor, voltage follows current, perhaps the PGOOD pin can be used to detect when the capacitor is >92% of target voltage. It is a logic pin and goes high at that point and goes low when that condition is no longer met.

Perhaps, it can be used to turn a logic level mosfet on, then use an RC circuit to keep the mosfet on for a little time until C2 is discharged to a level that will not operate the transmitter. Perhaps something like this:


R1 and C1 would provide that timing.

EDIT: I didn't check whether the PGOOD pin has a high reverse impedance. You may need a diode in that line to prevent the mosfet gate from discharging back through that pin instead of the RC circuit shown.