Timer chip provides an easy way to generate a pulse with a specific duration. The duration is determined by a timing resistor and capacitor.
The 555 timer chip contains two comparators and an internal set-reset flip flop. An external pin called RESET overrides the other inputs and can reset the chip at any time.
Threshold Comparator
A comparator takes two voltages and subtracts them to give you a logic output. It lives at the border between the analog world of continuous voltages and the digital world of timer chip 1’s and 0’s. The comparator has two input terminals, the non-inverting and inverting, which are typically labeled – and + on a circuit diagram. The inverting input is connected to the power supply, so it will be at a potential close to 0V and the non-inverting input will be at a potential equal to or greater than the reference voltage Vth.
The comparator has a built-in hysteresis, which prevents multiple transitions of the output when a noisy input signal changes over the threshold. Without hysteresis, the noise would create multiple transitions of the comparator output which could confuse the microcontroller trying to interpret the signal. Hysteresis sets a lower and upper threshold so that the signal must pass over both of them before the comparator output will change state.
The hysteresis is provided by the large resistance Rh which is placed in parallel with Ry, lowering the threshold of the comparator to about 2.3V. The comparator must pass below this threshold before the output will switch to a logic low. To test this, we can use a simple test circuit to simulate the operation of the comparator and see how it compares to a simple comparator with no hysteresis. The results are shown in Figure 7.
Trigger Comparator
This is an important part that converts the voltage signal to a binary output. Basically, this part works by monitoring the input and then firing when it is above a certain threshold. In order to do this, the comparator must be high speed as it needs to make decisions very fast. The comparator also needs to be immune to noise in order to prevent multiple output transitions.
To do this, the comparator uses a special type of positive feedback to create hysteresis. Hysteresis is a difference between the actual value of the output and the expected value. This is useful because it helps to eliminate the effects of noise or jitter on the signal.
For example, let’s say the signal is at 1.98V when it first reaches the comparator. As soon as it crosses this threshold, the non-inverting input of Q1 becomes saturated. As a result, the current through the shared emitter of Q1 and RE decreases in an avalanche-like fashion and the output changes to low.
Then, if the signal again crosses the upper threshold, the comparator changes back to its high state. This process repeats itself until the output is -VSAT. A comparator with precise thresholds and hysteresis is often designed using a pair of single-anode Zener diodes. This reduces the output levels and increases the PSRR (power supply rejection ratio). This configuration is ideal for a comparator that must be resistant to power supply fluctuations and input leakage currents.
Latch
A latch is a digital circuit that stores a binary digit (0 or 1). It can be used as a memory unit or as the building block for sequential logic circuits. It differs from a flip-flop in that it uses a level trigger rather than an edge one. In addition, a latch cannot change its output while the enable input is not active. It can only store the current state until the next rising edge of a clock signal.
Latches are simple digital circuits that can be easily implemented with basic digital logic gates. They use less power than other sequential circuits, and they can operate at high speeds. However, they are susceptible to noise and transient signals that can cause them to change their output unpredictably. This is known as a race condition in digital circuits.
Adding an inverter to a simple latch prevents the critical race and allows it to operate reliably. The inverter also makes it easier to set and reset the latch. This type of latch is ideal for applications that need to be able to switch between a high and low state.
Latch’s ePMS system is easy to use and supports bulk actions for efficiency. It also includes features to quickly revoke timer chip manufacturer or reassign access for tenants. This can be done manually or automatically using the ePMS system.
Output
The output pin connects to the positive rail of the power supply. It can either be set high or low by the inputs to the comparator or the threshold pin. When the voltage on this pin is less than 1/3 Vs, it makes the output go high (+Vs). This makes the timing capacitor discharge through the NPN transistor to a resistor connected to ground. The output is reset to its low state when the voltage on this pin rises above 2/3 Vs.
The comparators outputs are piped to the R and S inputs of an SR flip-flop. It will output a 1 when S is high and a 0 when R is high. The output of the flip-flop is then given to the output driving circuit to raise the current levels. This output can then be passed to the external output pin of the 555.
The 555 timer chip is one of the most useful ICs ever created and can be used to make all sorts of interesting projects. It can be used in Astable mode to create a square wave for flashing LEDs or making sounds or Monostable mode to produce a single pulse when triggered. It can also be used to control motors, alarms and other devices that require regular timing pulses. The list of possible applications is endless and the possibilities are limited only by the creativity of the maker.