Hi all,
I'm building a battery powered (1 AA and 5v step up) remote control for a radio. The Press To Talk (PTT) is controlled by an AVR driving a MAX323. Pulling the PTT line above 20k ohms turns it off, below 2k ohms to ground turns it on. The MAX323 just makes or breaks the PTT line to ground.
I want to add a fail safe PTT directly wired to one of the switch buttons on the remote so that if the battery is dead or removed the PTT can still be operated. The proposed button is also used for other functions (when the battery is good) so I have to disable the hardwired PTT to prevent activation.
I tried using a MAX325 that has one NC (normally closed) and one NO line, intending to open the NC line when powered and hoping that it would close when unpowered. But it was open when unpowered.
A NC reed switch would work but would draw too much power.
What I need is a simple circuit that when powered disconnects (or blocks by changing to high resistance) the hardwired PTT and when unpowered allows it to operate. Preferably requires _very_ low current to operate.
The PTT current being switched is tiny.
Thanks in advance
I'm building a battery powered (1 AA and 5v step up) remote control for a radio. The Press To Talk (PTT) is controlled by an AVR driving a MAX323. Pulling the PTT line above 20k ohms turns it off, below 2k ohms to ground turns it on. The MAX323 just makes or breaks the PTT line to ground.
I want to add a fail safe PTT directly wired to one of the switch buttons on the remote so that if the battery is dead or removed the PTT can still be operated. The proposed button is also used for other functions (when the battery is good) so I have to disable the hardwired PTT to prevent activation.
I tried using a MAX325 that has one NC (normally closed) and one NO line, intending to open the NC line when powered and hoping that it would close when unpowered. But it was open when unpowered.
A NC reed switch would work but would draw too much power.
What I need is a simple circuit that when powered disconnects (or blocks by changing to high resistance) the hardwired PTT and when unpowered allows it to operate. Preferably requires _very_ low current to operate.
The PTT current being switched is tiny.
Thanks in advance