ABSTRACT
Internal features To begin to understand how timer circuits operate, it is worth spending a few moments studying the internal circuitry of the 555 timer, see Fig. 12.1. Essentially, the device comprises two operational amplifiers (used as comparators – see page 166) together with an R-S bistable element (see page 188). In addition, an inverting buffer (see page 185) is incorporated so that an appreciable current can be delivered to a load. The main features of the device are shown in Table 12.1. Unlike standard TTL logic devices, the 555 timer can both source and sink current. It’s worth taking a little time to explain what we mean by these two terms:
(a) When sourcing current, the 555’s output (pin-3) is in the high state and current will then flow out of the output pin into the load and down to 0V, as shown in Fig. 12.2(a). (b) When sinking current, the 555’s output
(pin-3) is in the low state in which case current will flow from the positive supply (+Vcc) through the load and into the output (pin-3), as shown in Fig. 12.2(b).
