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About RelayRepRap

This project is about exploring whether it is possible to make a RepRap control system that can be made by a RepRap.

RelayRepRap on

Introduction Posted on Wed, June 01, 2016 22:00:28

I’ve temporarily or permanently moved updates about this project to

You can follow it here:

Vienna Museum of Technology

Inspiration Posted on Sun, May 01, 2016 23:02:10

I recently visited the Museum of Technology in Vienna. It’s well worth a visit. I particularly enjoyed the section containing physics experiments, many of which were interactive. There was a continuously sensitive diffusion cloud chamber – I’d never seen one before and it was fascinating to watch the particle tracks from background radiation.

There was also a section containing automated tools, including a numerically controlled drill and a Jacquard loom – see photos below.

Overall plan for a 2D plotter

Plans Posted on Sun, April 24, 2016 22:02:04

The PDF attached to this post shows a plan for a relay-based control system for a 2D plotter, driven by a punched tape. The plan has links to videos illustrating some of the ideas, and working implementations of some of the relay circuits.

I hope to make this over the coming months, initially using off-the-shelf relays, then eventually using easy-to-manufacture-at-home relays.

Information about relays

Relays Posted on Sun, March 13, 2016 22:35:26

Links to some good information about relays:

Handbook of Relay Switching Technique

J. Th. Appels, B. H. Geels

Definition of Relay Terminology

Electric Relays: Principles and Applications
Vladimir Gurevich

A 3D printed relay

Relays Posted on Sun, March 13, 2016 18:40:41

This is the most successful relay design that I’ve come up with so far:

View from underneath the relay, showing the strut that the pole pivots on:It’s an SPDT relay. The nail that is used as the core of the coil also serves as the normally open contact.

It’s printed all in one piece – this seems preferable to assembling
multiple pieces (as I did in an earlier version), because assembly
brings with it the potential for imprecise alignment of pieces.

moveable pole of the relay is mounted on a thin flexible strut. It
pivots on this strut – flexing the strut – as the relay is activated and
deactivated. In earlier designs I had this strut connected to the body
of the relay at only one end, but found that this gave too many degrees
of freedom (up/down, back/forth, pivot) and it became difficult to control the springiness of the

An OpenSCAD file for this relay can be found here.

A dab of epoxy glue is used to keep the pole nail and the normally-closed nail firmly in place. The coil nail fits tightly enough in its holder that it doesn’t require any glue – it can easily be removed for reuse in future design iterations.

The nails are galvanised clout nails with shaft diameter of 3mm and a head diameter of 9 or 10 mm. The heads are fairly uneven, and often the centre of the head is offset from the centre of the shaft. The coil and NC nails are 30mm long, the pole nail is 20mm long.

The coil is made from 1500 turns of 40swg enameled copper wire, it has a resistance of 35 ohms.

The operating voltage of this relay was found to be 3.1V, the release voltage 1.2V.

The resistance of the contacts is undesirably high – the normally closed contact has a resistance of 6.2 ohms, the normally open contact has a resistance of 1.2 ohms. (Measured with a 100 ohm load). Off-the-shelf general purpose relays typically have a contact resistance of less than 0.1 ohm.

The operate time of the relay is about 8ms, the release time about 9ms. The bounce time at the NO contact is typically < 1ms, the bounce time at the NC contact is much longer – the last bounce typically occurs at about 23ms after the removal of coil power.

I’ve operated this relay 60,000 times at 3Hz, switching low currents (about 1mA).

Toggle circuit using 3D printed relays

Relay circuits Posted on Sat, March 12, 2016 22:47:32

This video shows a toggle circuit made from 3D printed relays:

Here is the circuit diagram:

Here the downward pointing arrows represent 0V, the upward pointing arrows represent a voltage twice that required to activate a relay.

The toggle circuit failed fairly quickly (<100 operations) because
relay C is switching the coil of relay A and this causes arcing at the
normally open terminal of relay C which welds the contacts closed.

I found that this particular design of relay fails after a few 10,000s of operations even when switching non-inductive loads. Generally it fails by remaining closed after power is removed from the coil. I haven’t yet worked out what the failure mode is, but some possibilities are:

– PLA creep, reducing the force that pushes the moving pole back to the normally closed contact.

– Magnetization of the coil nail, causing the pole to stick on the normally open contact.

– The pole nail become loose.

A counter and clock divider circuit

Relay circuits Posted on Sun, February 28, 2016 23:55:50

This video shows a counter circuit that I hope will form the basis of a simple punched tape driven system for controlling the speed and direction of a pair of stepper motors.

The punched tape will have a simple mask for each motor for selecting the speed (1/1, 1/2, 1/3 or 1/4 of the clock rate) and direction. A third mask will be used for specifying the count of the modulo 12 counter on which the punched tape should advance to the next position.

Using relays to control a RepRap

Introduction Posted on Sun, February 28, 2016 00:01:20

The RepRap project has two parallel, overlapping aims – one is to make readily accessible low-cost 3D printers, the other is to make self-replicating 3D printers. The project was able to achieve the first goal because a RepRap is able to make many of its own mechanical parts: structural elements, gears, springs etc…

I’m interested in RepRap because I’m interested in self-replicating programmable constructors – i.e. machines that can be programmed to construct other machines, and which can construct themselves.

A difficulty in making a more completely self-replicating RepRap is the control system. RepRaps are controlled by microcontrollers, and microcontrollers are made by a complex process in expensive semiconductor fabrication plants.

I’m investigating whether a relay-based RepRap control system can be built using relays made from RepRapped parts, iron nails, and copper wire. My motivation for the project is largely curiosity about whether it can be done. I don’t expect a RepRap controlled by relays to have any practical value. And if the project doesn’t succeed, then finding out exactly why it won’t work is almost as good as succeeding.

Some forum posts about some of the ideas involved in this project can be found here:

VORBAT thread on RepRap developers forum.
Another thread including posts about VORBAT.

Some initial work that I’ve done includes:

– A stepper motor speed and direction controller made from off-the-shelf relays.

– A 3D printed relay that seems to operate reliably for 10,000s of operations (when switching low-current non-inductive loads at a switching rate of 10 cycles per second).

– A circuit diagram for a simple RepRap controller made from about 50 relays and a punched card reader.

Further details about these will be posted in due course.

At the moment it seems to me that the biggest hurdle in this project is likely to be the reliability of RepRapped relays when switching inductive loads (the coils of other relays). In order to be able to start printing useful objects, all of the relays in the controller need to operate about 10000 times at a speed of at least 5Hz without any of them malfunctioning.