Threshold activated circuit
Use a comparator to detect the treshold crossing. Apply this signal to both inputs of an XOR-gate, one directly, the other delayed. You can use a few gates to create the delay. The output of the XOR-gate will give a short positive pulse each time you cross the treshold. The width of this pulse is determined by the delay introduced by the gates. If you use slow logic like standard CMOS (CD4000 series) you'll get a few tens of nanoseconds per gate. If you need much longer pulses, then use this short pulse to trigger an MMV (monostable multivibrator).
Use an XNOR instead of an XOR-gate if you want negative pulses.