I am currently prototyping a custom intravenous medication delivery system. An AtTiny85 microcontroller is being used to control two L293D drivers, with 4 pins programmed to send a pulse train to both drivers and an additional pin being used to enable/disable each driver by way of an NPN/PNP logic system.
This leaves no remaining pins on my setup (the AtTiny85 is built into a 5V Trinket). I need to be able to externally control when this program is executed, and I have been doing so by shorting the Rst pin to ground, thus calling the setup() routine in which my algorithm is nested.
This is where problems arise: I have attempted to short to ground the following circuit, from which external feedback is provided from an NI I/O device:
However I have found this method to be unreliable - a 150 ms NI TTL pulse properly forces a reset on the AtTiny85 about 75% of the time, yielding the proper series of events. A proper reset fails to occur about 25% of the time, indicated by the lack of red light on the unit. Suggestions?