Safety Interlock Circuit for Vacuum Systems

Working in ultra-high vacuum (UHV) is a pain in the ass.  The idea is that by achieving such a high-quality vacuum (having something like twenty thousand billion times less gas atoms per unit volume flying around than in air), we can perform atomically well-defined experiments without all the airborne junk and hopefully learn something about physics at the atomic and molecular scale.

Ultra high vacuum system, named Dolores, on the left.  Me on the right.  We often fight.

But when things go wrong, there is a lot of down-time.  This post aims to share a simple electronic circuit that can help to relieve some of the pains associated with equipment failure.

Turbomolecular pump innards, from  US Patent EP0522603A1
A common type of pump used to achieve UHV is the turbomolecular pump, which looks very similar to an aircraft jet engine.  Its rotor turns really really fast (~1000Hz - yes, revolutions per second!) in order to transfer momentum to the gas particles in vacuum such that they are propelled toward its exit.  The turbo pump must itself be connected to a vacuum pump (known as a roughing pump, or forevacuum pump).  This ensures that the pressure throughout the turbo pump is sufficiently low to reduce viscous drag on the rotors.

Unfortunately, if something bad happens to the roughing pump, like it overheats, seizes, leaks oil, etc. the turbo pump might get damaged.  And in the case of an expensive magnetically levitated bearing pump, it might cost 8 weeks of research time and $25000 to replace.  Problems associated with roughing pumps have occurred twice in our lab in the last decade and have necessitated a turbo pump replacement.

Naturally, it would be nice to avoid such incidents in the future.  This is why I put together a simple, robust analog circuit to monitor the forevacuum pressure and shut down the turbo pump and close the vacuum valves if the pressure goes bad.  It only costs $50 to build, and I think I wrote a nice pedagogically rigorous report on "how to do it" so that you can learn something about electronics, and build it step-by-step.  Unfortunately, the Review of Scientific Instruments or Journal of Vacuum Science and Technology A won't even publish it as a "shop note" (yeah ok, I understand it's not "novel science", but it's useful!).

So, after a double editor rejection, I am unleashing it upon the internet.  Because somewhere, there is another poor suffering grad student working on a complex UHV system who might want to put together something like this as an investment toward their supervisor's wallet's well-being, their expedited graduation (freedom!), and their mental health.

Oh and by the way, it saved my ass when this happened:

KF flange that broke in the forevacuum line overnight, totally randomly.  Interlock to the rescue.

You'll find a very detailed description of the electronic circuit below, and more stuff including a parts list, below that.

We describe the design and operation of a simple analog circuit which can be implemented in various applications where a process should be shut off if a threshold value of an analog signal is reached. The threshold value, hysteresis, and trigger direction can be easily set. Judicious analog design ensures reliable operation, and versatile functionality is built in without the need for programmable microcontrollers. A PCB layout is provided along with a component list for quick assembly. We use the circuit in our lab to monitor forevacuum pressure to protect turbomolecular pumps and UHV pressure during sample annealing.

Safe interlocking of a UHV system is desirable to avoid damage from inevitable disruptions coming from equipment failure or power outages [1,2].  In the operation of our surface science UHV system, we have come across situations when it would be desirable to abort a processes automatically based on UHV or forevacuum pressure.

In this shop note, we discuss a circuit that was recently designed and implemented to monitor the pressure of a forevacuum line using the common analog output of a pressure gauge.  It protects a sensitive magnetic bearing turbomolecular pump against a possible rotary vane roughing pump failure (which has proved to be problematic twice in the last decade of operation, once causing several thousand dollars of damage).  If the forevacuum line pressure exceeds 1.5x10-2 mbar, the circuit described here is used to close the UHV and forevacuum valves around the pump, and the pump is stopped.  We also use this circuit to monitor the UHV pressure in our preparation chamber so that sample annealing is stopped if the pressure exceeds 6x10-8 Torr.  This allows us to run a filament overnight without worrying about vacuum malfunction.

The circuit is designed to close a relay during “normal” operation so that current can flow to the device or filament.  If the control signal passes a trigger value, the relay will open and halt the current flow.  The circuit can be configured for “normal” operation when the input voltage is lower than a trigger (normal low configuration), or higher (normal high configuration) than a trigger.  The trigger value is easily set by a potentiometer.  One resistor modifies the hysteresis trigger value for the return of the control signal to its normal state.  The circuit can also be configured for single-shot mode so that the relay will not close again until the user activates a “set” switch.  A bypass switch is provided to keep the relay closed regardless of the control signal value.  In addition, if the input signal is unplugged, the relay will open after a time delay.  This functionally simple, computer-less, analog solution is reliable, robust, and easy to construct.
The implementation of the aforementioned features will be described in detail with additional information in supplementary material (following this text).  In the following description of its design, we refer to the sections of the circuit that are labeled and circled by dashed lines in Figure 1.

Figure 1: Circuit diagram of the interlock circuit.  Circled sections are described individually in detail in the text.
In section A, the combination of R­IN and CIN assume the function of a low-pass filter for the control signal with corner frequency fIN = 1/(2πRINCIN) (assuming RIN << RDIS).  RDIS is a resistor used to disable the output by pulling the input in the opposite direction to the “normal” state in case the control signal is unplugged.  The disable function occurs after a characteristic time τIN = RDISCIN (the exact time also depends on the threshold level being used).  We use RIN = 100kΩ, CIN = 1µF, and RDIS = 100MΩ for a filter frequency of ~1.6 Hz and automatic disable time of ~100s.

The bypass switch, SWBYP, is implemented in section B with a DPDT switch used to impose a “normal” state regardless of the input voltage.  The other pole of the switch is used to turn on a LED to indicate the status of the bypass (RLED = 2.49kΩ for a current of ~4mA, assuming a 2V diode drop).

Section C contains the trigger setting potentiometer, VRT, and a hysteresis resistor RH.  The hysteresis means that the trigger voltage seen by the comparator changes based on the state of the output.  If R1 is the resistance between the potentiometer wiper and ground, and R2 is the resistance between the wiper and the +12V rail, then the upper and lower thresholds are given by Vupper = 12V·R1/(R1+R2||RH), and Vlower = 12V·R1||RH/(R1||RH+R2), where || denotes the parallel combined resistance.  For a 100kΩ potentiometer centered at R1 = R2 = 50kΩ, a hysteresis resistor of RH = 220kΩ, the upper and lower thresholds are ~6.6V and ~5.4V.  Test points TP1 and TP2 allow for easy testing of these thresholds using a multimeter.  This is described later.

In section D, a 2N3906 transistor (Q1) is driven by the output of the LM311 comparator (U1) through ROUT = 10k which powers a relay, RY1.  A 1N4004 flyback diode D1 protects against the back EMF generated when switching off the relay coil.  RY1 is used for the single-shot configuration.  If single-shot operation is not needed, RY1 can be omitted, and the point indicated by a star in section D should be connected by a wire to the corresponding star point in section E.

The output relay, RY2 is located in section E.  If the relay is energized, LED2 is illuminated (current limited by RLED2 = 2.5kΩ).  In the case that single-shot mode is not needed, the set switch, SWSET, can be eliminated.  The board mounted relay can switch a device (up to 8A) soldered directly to the pins labeled DEV in Figure 2(a).  If a higher current is needed, an off-board relay coil can be attached to the auxiliary outputs labeled RY2 in Figure 2(a).
Figure 2: Printed circuit board layout (actual dimensions: 36x122mm). (a) Copper traces shown in black with component overlay (normal low configuration shown); (b) assembled circuit board (normal high configuration shown)
Sections F1 and F2 set the normal high or normal low configuration of the board.  If “normal” operation should occur with a signal lower than the threshold, the circuit should be wired as per the normal low configuration shown in Figure 1, making the following connections: a-d, b-c, e-g, and f-h.  These correspond to the blue and red dashed connections in Figure 2(a), labeled a through h.  If normal high is needed, the connections should be made as: a-c, b-d, e-h, and f-g, which is shown in Figure 2(b).  These connections switch the direction in which the disable and bypass features work, as well as the order of the inputs to the comparator.  We use a normal low configuration for our forevacuum pressure monitors with a Balzers TPG300, and a normal high configuration for our UHV pressure monitor with a Lesker IG4400.

Power supply smoothing capacitors C­­1 = 22µF, and C2 = 0.1µF are included in section G.

We produced the single-sided circuit board shown in Figure 2(a) in our lab using Pulsar Toner Transfer Paper.

At the time of assembly, the circuit must be configured for  normal high or normal low operation.  If it is not being configured for single shot operation, the points marked by a star need to be connected by a wire and the optional components listed in the parts list (following this text) can be omitted.

The threshold should be set in the following way: 
  • Connect a multimeter to the test points labeled TP0 (common) and TP1 (threshold voltage). 
  • Connect a wire to the +in terminal block. 
  • Apply DC power to the circuit
  • By touching the +in wire to 0V or 12V (the screws on the power terminal block are ideal for this), the state of the relay will switch, and the multimeter will read the threshold voltages.
  • Adjust the potentiometer and RH value (we mount RH on IC pins for easy replacement) until the desired thresholds are achieved.
In summary, this simple, versatile interlock circuit is easy to build and configure for a variety of applications.  It has improved the safety and robustness of our existing UHV system, and has reduced the risks involved with roughing pump failure and unattended sample annealing.

[1] J. P. Saint-Germain, G. Abel, and B. L. Stansfield, J. Vac. Sci. Tech. A 4, 2391 (1986).
[2] J. A. Polta and P. A. Thiel, J. Vac. Sci. Tech. A 5, 386 (1987).


Standalone unit

We have integrated two of these circuit boards into a larger rack-mounted interlock panel to add pressure monitoring functionality. Another board was assembled as a standalone unit, shown below, for annealing samples. All the parts, except wires and mounting screws, are specified in the parts list.

Parts List

The following table summarizes the parts used in the interlock circuit, along with their corresponding Digikey Part # and the cost of the part in Canadian dollars at the time of writing.

The parts indicated as optional are those that are used for the single shot operation and can be omitted if un-needed.

Digikey Part #
Input Resistor
100k resistor
Input Capacitor
1uF, 63VDC, film capacitor
Disable Resistor
100M resistor
Bypass Switch
On-On DPDT switch
Bypass enable LED
Orange 3mm LED
LED current limiting resistor
2.49k resistor
Threshold potentiometer
100k trimmer potentiometer
Hysteresis resistor
220k resistor
LM311 comparator 8DIP package
Comparator output resistor
10k resistor
Relay driving transistor
Flyback diode (optional)
Set switch relay (optional)
SPDT 12V relay
Set Switch (optional)
On-Mom DPDT switch
Relay indicator LED
Green 3mm LED
LED current limiting resistor
2.49k resistor
Flyback diode (optional)
Main relay
8A DPDT relay for main connection
Power Supply Capacitor
22 uF, 50V, radial, aluminum capacitor
Power Supply Capacitor
0.1uF, 100VDC, film capacitor

Terminal blocks (x7)
2 position 3.5mm terminal block

Device connector
Panel mount banana jack

Device connector
Panel mount banana jack

Power connector
2.1mm ID, 5.5mm OD barrel connector

BNC input
Isolated BNC

Translucent Polycarbonate Enclosure 150x80x50mm

Power supply
12V, 0.5A, switching DC power supply

PCB Layout

The PCB layout and component layout is shown below.  A PDF of the design in actual size is available here. The PDF has 10 layouts on it so you can make a large toner-transfer page full of them!!!