The height of the waveform should not drop! Unless you connect this 5k simulating resistor across the collector and emitter while still have R3 intact. You should place the simulating resistor in series with R3. We still dont know how much current this biological tissue can handle or need so you cant really calculate the value of R3

 

The height of the waveform should not drop! Unless you connect this 5k simulating resistor across the collector and emitter while still have R3 intact. You should place the simulating resistor in series with R3. We still dont know how much current this biological tissue can handle or need so you cant really calculate the value of R3

The output of the circuit to the oscilloscope is achieved by placing the probes in the emitter and collector of the transistor. Is it right?

Because of that I put a 5K resistor (across the collector and emitter as well) simulating the resistance of the biological tissue. Makes sense for me because it is like instead of having the probes connected to the oscilloscope, I have them connected to the tissue with a resistance of 5K.

If I should place the simulating resistor in series with the R3, where should I get the output of this circuit to stimulate the tissue?

Thanks.

 

I think you are very confused, and heck, you are confusing me as well

What other output? Think of the biological tissue as an LED, what happen when you apply power to the LED? It lights up! Now if you want to know if the waveform remains the same then you would still probe at the transistor's collector and emitter

Ok, maybe I'm not good at explaining things, try reading this page: http://www.physics.udel.edu/~nowak/phys645/The_bipolar_transistor.htm

 

I think you are very confused, and heck, you are confusing me as well

What other output? Think of the biological tissue as an LED, what happen when you apply power to the LED? It lights up! Now if you want to know if the waveform remains the same then you would still probe at the transistor's collector and emitter

You gave the perfect example. I will ligth up the led and keep monitoring the square wave with the probes at emitter and collector. As soon as the led lights up, there is a drop into square wave amplitude (voltage). What should I do to prevent the drop?

For example: if i use a PIC to output a square wave with 5 V to LM317 and then use the LM317 to regulate this voltage down to 2.5 V, the Output of the LM317 will be a square wave with 2.5 V of amplitude. If I light up a led with this output, LM317 will keep the 2.5 V amplitude even with the Led ON. If I combine a LM317 to the transistor, can I ligth up the LED and keep the square wave unchanged?

Thanks.

 

First of all, you do not have to lower the voltage to light up an LED, (read some more about LED) you would need to regulate the current going through it, a typical LED uses around 20mA of current, so you would need that Ohm's law to calculate the value for R3 so that it would limits the current running through the LED to ~20mA.

About voltage drop, how much did it dropped?

 

First of all, you do not have to lower the voltage to light up an LED, (read some more about LED) you would need to regulate the current going through it, a typical LED uses around 20mA of current, so you would need that Ohm's law to calculate the value for R3 so that it would limits the current running through the LED to ~20mA.

About voltage drop, how much did it dropped?

I did set the DC Variable Source to 10 V and took a picture of the oscilloscope. Then, I added a resistor of 4.7K across the probes of the oscilloscope and took another picture.

The voltage dropped to half.

 

How many time do I have to say this? 'You have to connect that resistor in series with R3'! It cannot be applied to the transistor's collector and emitter.

Lower the frequency to around 10Hz and then replace R3 with a 330 Ohm resistor, connect an LED in series with R3, change the supply voltage to 5V an observe the blinking LED, probe and see

 

How many time do I have to say this? 'You have to connect that resistor in series with R3'! It cannot be applied to the transistor's collector and emitter.

Lower the frequency to around 10Hz and then replace R3 with a 330 Ohm resistor, connect an LED in series with R3, change the supply voltage to 5V an observe the blinking LED, probe and see

I lowered the frequency to 5Hz to be easier to see the LED blinking, replaced R3 with a 330 ohm resistor, changed the supply voltage to 5V and probe from the emitter and collector.

The LED was blinking but the square wave was affected. See photos.

Thanks.

 

Good, now we are getting somewhere! 5.0V - 3.6V = 1.4V, that is expected since an LED is like a diode, it drops 1.4V voltage.

Now replace the LED with a 5k resistor, probe and see

 

Good, now we are getting somewhere! 5.0V - 3.6V = 1.4V, that is expected since an LED is like a diode, it drops 1.4V voltage.

Now replace the LED with a 5k resistor, probe and see

I used Proteus to draw the different circuits and attached a photo of the oscilloscope below each circuit to show the results.

My only concern is regarding the last photo (Circuit and Scope_D.jpg). In my opinion, this is the best location to place the probes to see what is happening for real, since the 5k resistor is simulating my real target that will be a biological tissue.

Take a look.

Thanks.

 

Any suggestion?

Thanks.

 

Why are you using I1 (a current source) in your sim? Shouldn't the Vreg U1 be powered by a voltage source?
Your scope -ve probe is probably grounded, so connecting it where you show in the last pic will upset the operation of the circuit.

 

Why are you using I1 (a current source) in your sim? Shouldn't the Vreg U1 be powered by a voltage source?
Your scope -ve probe is probably grounded, so connecting it where you show in the last pic will upset the operation of the circuit.

Alec_t,

I misused the symbol. In the real circuit it is a voltage source.

The idea behind this circuit is to have a electrical stimulator for biological tissues that have a variable resistance (something around 1 and 7kohm). I use the PIC to generate a square wave and since PIC out cannot be high voltage, I get the PIC output and feed it into a BC548 NPN transistor in order to have the square wave with higher voltage. However, I don't know exactly where I should get the output to the biological tissue and how can I simulate the tissue with a resistor.

Please I need some ligth on this because I am kind of lost.

Thanks.

Dan07.

 

Your sim circuit is fine, but always use the scope with its -ve probe on circuit ground. Use the +ve probe to measure voltage at various points. You can then add/subtract the measurements to calculate the voltage across components such as your 5k resistor.
If your bio tests are done on a live body then make sure your kit is powered from batteries, NOT the mains, and ensure that probes do NOT penetrate the skin.