The 555 timer, introduced in the 1970s, has proven popular with electronic engineers and hobbyists. As with any electronic device, it can develop problems, though circuits built using the 555 tend to be simple and therefore straightforward to diagnose. Other than a DC power source, the 555 depends on a timing network consisting of a few resistors and capacitors. If the power supply works, and if the external components remain in good shape, the 555 timer integrated circuit itself may have gone bad. Since you can buy these timers for $1 or less, replacement is quick and inexpensive.
Power on a digital multimeter, and set its function knob to read DC volts. With the power to the circuit on, touch the positive meter probe to the IC’s pin 8 and the negative probe to pin 1. The meter’s display should read between 4.5 and 16 volts.
Examine the resistors and capacitors in the timing circuit. Look for burn or scorch marks, dents, cracking and bulging. A bulged capacitor usually indicates part failure. Replace any parts you find damaged.
Examine the 555 timer circuit’s connections. Look for broken wires and bad solder joints. Look for solder blobs and other conductive debris that may short two unrelated signal paths together. If you see an obvious connection problem, carefully resolder it with a 30-watt soldering iron.
Remove the 555 timer from the circuit. If it’s in a socket, carefully pry it out with an IC puller or small slotted screwdriver. If the 555 is soldered in, desolder it by heating the solder pads briefly and then drawing off the melted solder with a desoldering pump. Pause a few minutes every two or three pins to let the chip cool off.
Set the multimeter to read capacitance. Touch one meter probe to each lead on the main timing capacitor. Read the capacitance on the meter display. It should agree with the capacitance value printed on the capacitor to within about 20 percent.
Replace the 555 timer IC if the connections and other components appear in good working order.
The maximum operating voltage varies by version of the 555 IC, so check the manufacturer’s data sheet if you’re not sure. New ICs carrying the 555 part number have better electrical specifications than older parts. Other than that distinction, you can consider the parts identical for the purpose of replacement.