By eliminating the need for external components, the 555 Propagation Delay Oscillator transforms the 555 into a self-contained oscillator, operating typically within the 1 to 2.5mHZ range. In this configuration, the need for a timing capacitor is eliminated, creating essentially a simplified version of the ring oscillator. Although its practical applications may be somewhat constrained, it serves as an excellent experiment to deepen one’s understanding of the versatile 555 timer and electronics in a broader sense.
Types of oscillators
The L-C oscillator utilizes an inverting amplifier that introduces the output signal back into the input with an inverted phase relationship, specifically 180° out of phase. This setup employs either an LC tank circuit or an equivalent crystal. In this scenario, the oscillator’s frequency is determined by the resonant frequency of the tank circuit.
A relaxation oscillator, a commonly used type, has a basic form involving a neon discharge tube connected across the capacitor within an R-C circuit. Here, the oscillator’s frequency hinges on both the rate at which the capacitor charges and the upper and lower voltage thresholds of the gas discharge tube.
The 555 timer also falls under this category due to its incorporation of an R-C circuit and the presence of upper and lower voltage thresholds. Similarly, a 74HC14 can be operated (made to oscillate) in this mode if its output is linked to an R-C circuit where the capacitor loops back to its input.
Propagation delay oscillator
Yet another oscillator type worth mentioning is what I’ll dub the “propagation delay oscillator.” This oscillator leverages the inherent propagation delay present in all logic devices to create a straightforward oscillating circuit. In the past, this approach wasn’t practical with a single inverter due to the exceptionally high speed of logic devices. You may have experienced inadvertently connecting an output to an input, causing some devices to oscillate at around 100mHZ due to a 2 * 5nS propagation delay, generating excess heat and potentially damaging the component.
However, the 555 propagation delay oscillator employs just a single 8-pin inverting 555 chip, and its frequency is low enough to avoid such issues. This concept bears some resemblance to the ring oscillator, which uses around five logic inverters connected in series.
It’s important to note that the 555 is not precisely a logic device, as it was never intended for operation with low propagation delays. While datasheets typically do not specify the maximum frequency, one source indicates a maximum frequency of 360kHZ, though TI claims 2mHZ for the TLC555. Due to its relatively slow operation, the frequency generally remains below 2.5mHZ, and the device does not self-destruct when operated without a timing capacitor.
Circuit 1 is slower and the frequency may be pulled (adjusted) by approximately ±3%.
Circuit 2 is the fastest and simplest. Note that the open collector output (pin 7) is very slow and may not output a valid signal –this feature may be neglected if desired.
Simplicity, in this case, comes at a cost –frequency of oscillation is a crap shoot and adjustment is limited.
- informative and fun experiment
- wherever a cheap oscillator may be required with minimum component count
- test fixture for grading devices –selecting fastest (or slowest) devices
- means of generating a fixed RF signal for spot checking radio receivers