This photo-indicator allows you to determine when the ambient light intensity is sufficient to obtain a good photo, from a previously established level. Experimentally, the sensitivity of the device can be adjusted to indicate the minimum necessary for a given machine to obtain a good photograph. Thus, in cloudy weather, with the indication of the green LED, there is no danger of losing the photo, as we will be guaranteed the minimum necessary for a good photo. The complete circuit of the photo-indicator is shown in the figure. The LDR is the sensitive element, which is connected to a power supply (9 V battery), an adjustment pot and a relay. The relay triggers two LEDs, the red in the normally closed position and the green in the normally open position. Then, adjusting the system to a certain level that, which turns out to be the minimum to obtain a good photo, we have two possible behaviors: if the light level is lower, the relay does not close its contacts and the red LED remains lit. If the light unit exceeds the minimum set, the relay is activated and the green LED lights up.
