Messrs. Schmitt, Trigger and APC (and other stories)


Just for the hell of it, I wondered if it was possible to substitute the 555 timers in an Atari Punk Console (APC) with some other component, or components.

After seeing some Schmitt-trigger inverters (CD40106) used in oscillator circuits, see Swingers, I thought that I would come up with an alternative design.

See also


Basic design

An APC consists, basically, of two multivibrators, an astable and a monostable.


Having seen an astable schematic based around a Schmitt-trigger inverter,

Inverting Schmitt trigger astable oscillator
Inverting Schmitt trigger astable oscillator

From BASIC ELECTRONICS COURSE – THE SCHMITT TRIGGER (Page 49), the frequency of the oscillation is given by 1.2/(RTCT), this is actually incorrect. See also Electronics Tutorials – Waveform Generators, which states that the frequency is given, approximately, by 1/1.2RTCT.

The Schmitt trigger can produce excellent astable square wave oscillators with just a RC circuit connected around it for feedback. As the capacitor charges and discharges via the feedback resistor, the voltage across its plates reaches the upper and lower threshold trip points with the inverters upper threshold point at around the 65% (2/3rds) of the supply voltage while the lower threshold point occurs around 35% (1/3rd) of the supply voltage.

However, using different logic families whether CMOS or TTL (74LSxx, 74HLSxx, 74HCTxx, etc) will result in different upper and lower trip points resulting in differing operating frequencies and mark-to-space ratios of the output timing waveform. Generally the error in the oscillating frequency for different logic sub-families is not a problem especially at higher frequencies, but can be anywhere from 1.2RC for basic TTL gates up to 1.7RC for fast CMOS types depending on the logic family used.

Note that RT would be replaced with a potentiometer, thus allowing the frequency to be changed.

See also help related to frequency of schmitt oscillator.


I then only needed to find a monostable based around the same device.

A simple google came up with this schematic, from 40106 Monostable:

40106 Monostable
40106 Monostable


40106 Monostable
When the button is pressed it discharges C1. This brings the input of IC1a to 0V
making the output high. When the button is released, C1 begins to charge through
VR1. As C1 charges, the potential at the input of IC1a rises exponentially up to the
supply voltage. When it passes the upper trip point it is interpreted by IC1a as a logic 1 and the output switches to logic 0 where it remains until the push button is pressed again.
The time duration of the output pulse is of the order VR1 x C1.
Note: Pulse timing begins after the push button is released. This might be undesirable in some situations.

Also, worth a note is problems with a 40106 monostable

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s