# How to create a trigger event with a pulsed DC current.

Thread Starter

#### jmorris23

Joined Jun 4, 2021
4
Hello, I'm running an experiment using perturbation goggles. I'm trying to obtain a trigger event from the occurrence of a perturbation. The device outputs a voltage from two wires. Since the amplifier that I am trying to record the voltage from can only take <5V, I am using a differential op-amp setup to only output the difference between the two input voltages. The problem is that the device is outputting a pulsed dc. (verified using an oscilloscope). The trigger events result in pulses that are higher voltage. The amplifier reads triggers as an input voltage or voltage change. So when I plug the circuit and device into the amplifier, I get trigger events at every single pulse (both the low constant voltage pulses and the actual trigger higher voltage pulses).

I figure that if I google solutions long enough I will get something, but I thought I'd ask you guys for some help. I'm aware of reservoir capacitors but they do not completely smooth the signal, so I fear that there will still be trigger events occurring at each pulsed dc still. I'm thinking of combining the capacitor and a relay to open and close the circuit at the larger trigger event. I think that the relays work based on larger shifts in voltage and so the capacitor will smooth out the pulsed DC and then only the larger triggers will be represented. Since my analysis relies heavily on the timing of the events, I was wondering if the capacitor will affect my timing.

I'm really just looking for any and all ideas here. Thank you in advance!

#### Papabravo

Joined Feb 24, 2006
21,306
I'm having a very hard time visualizing what you are talking about. A verbal description is simply inadequate to describe any circuit. I would like to help you but I need you to provide a schematic diagram of your circuit, hand drawn, annotated and scanned if necessary. Otherwise you may need to wait for someone more intuitive than myself who can parse your prose.

Thread Starter

#### jmorris23

Joined Jun 4, 2021
4
I'm having a very hard time visualizing what you are talking about. A verbal description is simply inadequate to describe any circuit. I would like to help you but I need you to provide a schematic diagram of your circuit, hand drawn, annotated and scanned if necessary. Otherwise you may need to wait for someone more intuitive than myself who can parse your prose.
Hey, thanks for replying. I don't necessarily have an issue with the circuit. The pulse is being generated from the device (the goggles). The manufacturer made it that way. I'm really just looking for ideas that would bypass the pulses in the naturally occurring dc current coming from my voltage source. I'm not really looking for specifics, but here's my little schematic.

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#### Papabravo

Joined Feb 24, 2006
21,306
Hey, thanks for replying. I don't necessarily have an issue with the circuit. The pulse is being generated from the device (the goggles). The manufacturer made it that way. I'm really just looking for ideas that would bypass the pulses in the naturally occurring dc current coming from my voltage source. I'm not really looking for specifics, but here's my little schematic.
What is the part number of the amplifier? What are the power supply voltages? Are the supplies bipolar or unipolar?
Are inputs V1 & V2 referenced to your amplifier ground? If not, what is their common mode range?
Is R2 connected to your amplifier ground?
Do all three resistors have the same value? What is that value?
I did ask for annotated. Did you not understand the meaning of annotated?
I think you do have an issue with the circuit because it is not giving you what you expect.
I'm trying to help you, but you keep putting up roadblocks.

#### AnalogKid

Joined Aug 1, 2013
11,196
Your question is missing some things ...

What are perturbation goggles? What are the output voltage characteristics? Steady state (resting DC level)? Peak or Peak-to-peak during a "perturbation" (whatever that is)? Output source impedance or current capability? Frequency components of the output signal? How many wires to the goggles? Do the goggles require a power source? If yes, what is it?

etc.

You have to help us out. Assume that we know everything in the universe about electronic circuit and signal conditioning design, but absolutely *nothing* about what you know, what you are trying to do, and what "perturbation goggles" are.

ak

#### sparky 1

Joined Nov 3, 2018
784
Finding a mysterious movement beyond what sensor technology defines.
The word perturbation makes me think someone is feeling a need to go beyond the established terms
used to describe events in an electromagnetic field. https://arxiv.org/abs/hep-ph/9607283
An example where the term perturbation is being used to descibe a type of motion within a given context compared to
an object " perturbation goggles " a contrivance known by few with no introduction.

We study electrical science having respect for established terminology. An example of a statement that can be referenced to a subject.
A magnetic field can be sensed with a hall effect sensor.
You can also search for 3D sensors.

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Thread Starter

#### jmorris23

Joined Jun 4, 2021
4
Ok, so sorry for all of the confusion. I have never done one of these posts. I'm also trying my best to convey my issue. Here goes attempt 2:

1. Perturbation goggles are just a form of strobe goggle that I am using in my experiment. They block a person's vision (when the voltage output of interest is applied, this is what I am collecting as a trigger event). The goggles pseudo-randomly go completely black for about 1.5 seconds.
2. There are three wires coming from the goggles: ground, lf1, and lf1-com (Now, I'm not positive what those two mean because the manufacturer is not very good at responding to my emails and this device setup was made specifically for our lab to use). The two input voltages V1 and V2 are the lf1 and lf1-com.
3. The reason I did not include all of the information about the resistors and such is because the circuit is only taking my V1 and V2 and giving me the difference so that I don't supply too much current/voltage to the amplifier. I didn't think it had anything to do with my issue. Right now we are experimenting with resistors to get the safest number's possible, we did R1=R2= 1k Ohm and also R1=5.1K Ohms and R2=3k Ohms. We get a safe output voltage of 3V for the open lens (no trigger) state and about 4.32V when the lens is dark. This is in the range that the amplifier can accept (<5V).
4. I am only using the amplifier to record the data into a usable time-series signal that will allow me to synch other data that I am recording from other synchronized amplifiers. https://brainvision.com/products/liveamp-8-16-32/. I am recording electrical brain activity from 108 channel electrodes into a few separate amplifiers that are synchronized. I need the trigger events from my goggles in order to observe the brain activity that is going on around these events (darkness).
5. Since the product I am working with (the BrainVision system) expects a TTL signal as a trigger event, I have to take the signal from my goggles, reduce the amount of voltage supplied to the amp, and turn it into a form that is usable for my device as a trigger marker.
6. Since the goggles output a voltage that looks like the one shown in the attached files, (or a pulsed dc) I am having a rough time using this device to mark events in my data. (The second image shown does not represent the output when I use the differential op-amp circuit, it is an image supplied by the manufacturer). But you do see what I am talking about, the pulsing? This pulsing occurs when the goggles are on and open, and the pulsing voltage is increased when the goggles are closed, but the pulsing still persists.
7. The goggles are wireless so they have an internal battery supply.

So my question to all of you circuit experts, is how can I clean up this signal? I am under the impression that I should use a relay to open and close the circuit at the necessary trigger event. I would have to make something that reads the signal from my goggles and outputs a voltage (or even stops the voltage flow) to the amplifiers at the onset and offset of the "blocked view". I need assistance figuring out how to ONLY record the onset of the voltage increase and the end of the increase -not each pulse in the current. The system will mark these as events.

As a biomedical Ph.D. student who did not expect to be designing circuits at all, I sincerely apologize for my ignorance and thank you for being patient with me.

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Thread Starter

#### jmorris23

Joined Jun 4, 2021
4
** Update** after much research, I think that my goggles are actually providing a two-wired AC. So in order to get a clean DC, I need to use a capacitor in series with a resistor before the load (or output). Then I need to use a solid-state relay? Does this sound far off?

#### eetech00

Joined Jun 8, 2013
4,044
Ok, so sorry for all of the confusion. I have never done one of these posts. I'm also trying my best to convey my issue. Here goes attempt 2:

1. Perturbation goggles are just a form of strobe goggle that I am using in my experiment. They block a person's vision (when the voltage output of interest is applied, this is what I am collecting as a trigger event). The goggles pseudo-randomly go completely black for about 1.5 seconds.
2. There are three wires coming from the goggles: ground, lf1, and lf1-com (Now, I'm not positive what those two mean because the manufacturer is not very good at responding to my emails and this device setup was made specifically for our lab to use). The two input voltages V1 and V2 are the lf1 and lf1-com.
3. The reason I did not include all of the information about the resistors and such is because the circuit is only taking my V1 and V2 and giving me the difference so that I don't supply too much current/voltage to the amplifier. I didn't think it had anything to do with my issue. Right now we are experimenting with resistors to get the safest number's possible, we did R1=R2= 1k Ohm and also R1=5.1K Ohms and R2=3k Ohms. We get a safe output voltage of 3V for the open lens (no trigger) state and about 4.32V when the lens is dark. This is in the range that the amplifier can accept (<5V).
4. I am only using the amplifier to record the data into a usable time-series signal that will allow me to synch other data that I am recording from other synchronized amplifiers. https://brainvision.com/products/liveamp-8-16-32/. I am recording electrical brain activity from 108 channel electrodes into a few separate amplifiers that are synchronized. I need the trigger events from my goggles in order to observe the brain activity that is going on around these events (darkness).
5. Since the product I am working with (the BrainVision system) expects a TTL signal as a trigger event, I have to take the signal from my goggles, reduce the amount of voltage supplied to the amp, and turn it into a form that is usable for my device as a trigger marker.
6. Since the goggles output a voltage that looks like the one shown in the attached files, (or a pulsed dc) I am having a rough time using this device to mark events in my data. (The second image shown does not represent the output when I use the differential op-amp circuit, it is an image supplied by the manufacturer). But you do see what I am talking about, the pulsing? This pulsing occurs when the goggles are on and open, and the pulsing voltage is increased when the goggles are closed, but the pulsing still persists.
7. The goggles are wireless so they have an internal battery supply.

So my question to all of you circuit experts, is how can I clean up this signal? I am under the impression that I should use a relay to open and close the circuit at the necessary trigger event. I would have to make something that reads the signal from my goggles and outputs a voltage (or even stops the voltage flow) to the amplifiers at the onset and offset of the "blocked view". I need assistance figuring out how to ONLY record the onset of the voltage increase and the end of the increase -not each pulse in the current. The system will mark these as events.

As a biomedical Ph.D. student who did not expect to be designing circuits at all, I sincerely apologize for my ignorance and thank you for being patient with me.
Hi

Trying to understand your description...

A trigger event occurs when pulses are generated from the goggles. Right?
So...what is needed is a circuit that detects when pulses are occurring, or not occuring? and provides a output voltage to an amplifier input?

A Missing Pulse Detector circuit (MPD) can be used to detect when pulses are occuring. What type of signal should be output from the MPD when pulses are detected?

In addition,
The manufacturer picture shows what looks like a bi-polar square wave. Basically, a bi-polar square wave is a signal that transitions from a positive voltage to negative voltage and repeats. Is this correct? and is this the signal that is output from the goggles? We will need to know the voltage levels of this signal. If this is the signal output from the goggles, then we'll need a different type of circuit.

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#### LesJones

Joined Jan 8, 2017
4,226
Two suggestions.
1 - Trigger a monostable on all transitions then sample the signal level when to monostable times out. (So in the light part of the waveform the sample value would be zero.) If the sample is above +5 volts or below -5 volts then you are in the dark part of the waveform. If it is near zero then you are in the light part of the waveform. To save having to check for positive and negative values you could pass the signal through an active rectifier before sampling it.

2 - Pass the signal through an active rectifier circuit and a low pass filter. (To average the value.) During the dark part of the waveform the output of the filter would be a bit above 5 volts. during the light part of the waveform the value would be close to zero as the average of the fast spikes in the light part of the waveform would be quite low.

Les.