555 trigger amplitude slightly too large to trigger

Question

I have the following circuit which is designed to take a longer pulse (on the order of miliseconds) from a microcontroller (source v3 below) and use two 555 timers in cascade to output two consecutive smaller width pulses (ranging from 0.5us to 10us and individually adjustable through a pot [not shown in the schematic below]). These two consecutive pulses are triggered from this longer pulse. The output pulse from the second 555 timer should follow after, when the output from the first timer goes low. I've used RC differentiators between the first BJT inverter and between the two 555 timers to avoid multiple triggering of the timers.

The simulated circuit below shows from top to bottom the input pulse, the output after the BJT inverter, the output after the first edge detector (555 trigger input number 1), the output of the first 555 timer, the output after the second edge detector (555 trigger input 2) and the second 555 timer output.

I've built this circuit and the output of the first 555 timer produces a satisfactory pulse however the input to the trigger of the second 555 timer with edge detection reaches a value just short of the 1/3VCC value required to trigger the second timer. In simulation it dips below this 1/3VCC value and the second 555 timer triggers correctly however in practice this does not happen as shown in the oscilliscope reading below. The lowest value it reaches is 2V.

Oscilliscope measurement at the input to the second 555 timer. (node titled in1 in the schematic above)

I have tried adjusting C4 and R9 (edge detector time constant) to get this value lower but it affects only the discharge rate rather than the amplitude. Connecting the top of the second edge detector to a lower voltage such as 3v rather than 5v results in a lower trigger value however I don't want to use a seperate lower source to solve this. I'm looking for a simple way to drop this trigger voltage by 500mV or so, so that the second timer will trigger.

edit

• I'm actually using an NE556 IC instead of two NE555s.
• The outputs of the 555 timers go into unity gain buffers with an op-amp.

Answer 1

Trigger activates at \$1\over3\$ Vcc there's no need to pull up all the way to VCC; pull to \$1\over2\$ VCC instead.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

You could try a 10-100k pull-down resistor to ground on the output of the first and the trigger of the second. You may need to play with a few values to get the correct response. Depending on the 555 type the output can be 2 volts below vcc. Although most diagrams I have seen with one 555 triggering another are just directly connected between out and trigger.

Answer 3

I would suggest replacing the capacitors, same resistors and diodes at the Trigger inputs of LM556.