555 Propagation Delay Oscillator Schematic

555 Propagation Delay Oscillator

With no external components, a 555 can be turned into an oscillator that runs typically in the range of 1 to 2.5mHZ –or who needs a timing capacitor? It is essentially a stripped down version of the ring oscillator. While useful applications may be limited, it makes a great experiment to further enhance both one’s knowledge of the venerable 555 and electronics in general.


555 Propagation Delay Oscillator Schematic

Types of oscillators

The L-C oscillator employs an inverting amplifier that feeds the output signal back into the input with an inverted phase relationship (180° out of phase). Such uses an LC tank circuit or crystal equivalent. In this case, the frequency is determined by the resonant frequency of the tank circuit.

The relaxation oscillator is a common type with the simplest being a neon discharge tube connected across the capacitor in an R-C circuit. In this case, the frequency is determined by the charge rate of the capacitor and the upper & lower voltage thresholds of the gas discharge tube. The 555 also falls into this category because it has an R-C circuit and upper & lower voltage thresholds. A 74HC14 can operate (oscillate) in this mode as well if the output drives an R-C circuit with the capacitor connected back to its input.

Propagation delay oscillator

Still another type is the propagation delay oscillator (I’m coining a new term here). Since all logic devices have inherent propagation delay, this property can be applied to make a simple oscillator. In the past this has not been practical with a single inverter due to the extremely high speed of logic devices –perhaps you (like I) have destroyed logic devices by inadvertently connecting the output to the input –such may oscillate at perhaps 100mHZ due to a 2 * 5nS propagation delay and destroy itself by the heat generated. The 555 propagation delay oscillator uses only one 8pin inverting 555 chip, but the frequency is low enough to prevent smoke. This is somewhat similar to the ring oscillator that uses (5) or so logic inverters connected in series.

On the other hand, the 555 is not exactly a logic device as it was never intended to operate with low propagation delays. One source indicates a max frequency of 360kHZ –datasheets generally do not indicate the maximum frequency, but TI claims 2mHZ for the TLC555. Due to the slow operation, the frequency is generally less than 2.5mHZ and the device does not self-destruct when operated without a timing capacitor.

Two circuits

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.


  1. informative and fun experiment
  2. wherever a cheap oscillator may be required with minimum component count
  3. test fixture for grading devices –selecting fastest (or slowest) devices
  4. means of generating a fixed RF signal for spot checking radio receivers


555 Propagation Delay Oscillator Data

555 Propogation Delay Oscillator Oscillographs


Undocumented words and idioms (for our ESL friends)

crap shoot –idiom –gamble –results unknown beforehand

For the future

Relaxation oscillator schematic assortment

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