# How to detect a fluctuating voltage signal

#### Nick Long

Joined May 12, 2020
75
Hello! I want to design a circuit that can detect a fluctuating voltage signal. The specific function I want to realize is described as follow:
As shown in the figure below, I want to output a high voltage when the peak value (in the blue circle) of the signal (the red line) is lower than a threshold. For example, when the peak value in the blue circle is lower than 0.05V, a component, like the voltage compartor will output high. I hope the circuit realization is as simple as possible.
Do anyone have a good idea to achieve that? Thanks.

#### MrChips

Joined Oct 2, 2009
30,434
Yes it can be done.
But before we run around in circles, tell us exactly what you are doing. We want to hear more than just when a peak is lower than a threshold. What is the application?

#### Nick Long

Joined May 12, 2020
75
Yes it can be done.
But before we run around in circles, tell us exactly what you are doing. We want to hear more than just when a peak is lower than a threshold. What is the application?
I want to monitor the Glucose concentration. This voltage signal is the output of a Glucose concentration sensor.

#### MrChips

Joined Oct 2, 2009
30,434
Usually one wants to detect when a peak exceeds a threshold. This can be done using an analog comparator.
Your graph does not represent glucose concentration to me.
I would expect it to look more like this graph:

or this:

#### Nick Long

Joined May 12, 2020
75
Usually one wants to detect when a peak exceeds a threshold. This can be done using an analog comparator.
Your graph does not represent glucose concentration to me.
I would expect it to look more like this graph:

View attachment 241865

or this:

View attachment 241866
It is the output voltage curve from the Glucose concentration sensor, not the Glucose concentration curve. The difficulty is that the voltage signal fluctuates with time. The Glucose concentration will cause the peak value of the output voltage of the sensor to vary. Note that the fluctuation always exists, the change is mainly the peak value. Different Glucose concentrations correspond to different peak values.

#### Alec_t

Joined Sep 17, 2013
14,201
There are three main stages:
1) Amplification
2) Peak-detection
3) Threshold comparison
Stage 1 is analogue only, but stages 2 and 3 could be done with a micro-controller.

#### Nick Long

Joined May 12, 2020
75
There are three main stages:
1) Amplification
2) Peak-detection
3) Threshold comparison
Stage 1 is analogue only, but stages 2 and 3 could be done with a micro-controller.
Yes. Do you know how to achieve peak detection? I don't want to use the ADC to find the maximal value.

#### Alec_t

Joined Sep 17, 2013
14,201