The simplest option is to connect an LDR and resistor (try about 10k) as a voltage divider, then connect this to the reset input (555 pin 4). For example: LDR between +Vs and pin 4, 10k between pin 4 and 0V. Swop LDR and 10k to invert the switching action. However, this may not work well because the 555 has quite a low impedance reset input.
A better method is to use a second 555 as a buffer circuit and connect the output of this to the astable 555 reset. For a circuit diagram and further information please see www.kpsec.freeuk.com/555timer.htm (555 circuits)
and www.kpsec.freeuk.com/vdivider.htm (voltage dividers).
A transistor switch could also be used to connect the LDR to the reset input, please see www.kpsec.freeuk.com/trancirc.htm |
