E-Stop Circuit Upgrade: Schmitt Trigger Inputs

To the SN74HC7001N Schmitt trigger AND gates I ordered arrived. These hysteresis on these should solve the flaky e-stop problems I’ve been having. As noted in previous blog posts, the wireless e-stop would sometimes inexplicably go into a stopped state. The problem seems to be that the xBee modem logic outputs don’t reach 3.3v. In order to test the Schmitt trigger solution, I built the test circuit shown below:

Results

• With all switches closed, the relay closes and the arm is enabled.
• With any of the input switches (cRIO, Stop button, IRC5) open, pin 8 of the 74HC7001 goes low, corresponding to a drivetrain stop.
• When the cRIO or Stop button input is open, the IRC5 stop relay opens, corresponding to an arm stop.
• Then the IRC5 input is disconnected, there is no effect on the IRC5 relay, as intended. This is to prevent a possible race condition where the IRC5 could latch in the disabled state.
• I did not test the driver circuit for the drive base solenoid, but I previously tested this part on October 12.

AND Gate Drop-In

With the Schmitt Trigger AND gate proven to work in the test circuit, I reworked the E-estop receiver circuit board with one of the 74HC7001N ICs to see if it would fix the problems. On my first attempt, with pins 1 and 2 high, pin 3 was still in the low state, which suggested that I had destroyed the IC somehow.

I tried again, this time installing the 74HC7001N in a DIP14 socket instead of soldering directly to the board. I determined that the previous failure was not due to the IC failing, but because the input I was using to simulate the cRIO’s 5V logic was only 3.3v. Although 3.3v would have been enough to trigger the input, the protection diode circuit on the E-stop board was dropping it down to 1.65v on pin 2 of the 74HC7001, which is not high enough. With the 74HC7001N installed in place of the previous AND gate IC, the E-stop seems to work properly. The next step is to test it on ABBY.