How to 'shift' the voltage of a signal

[MikeBrady]

Hi,

This is actually a followup question to another thread...but a few weeks have gone by and it looks like the thread is dead. That's why I'm asking the question as a new post.

I am measuring the output from an IR receiver ZD1952 (if interested, further details can be found athttps://www.jaycar.com.au/products_uploaded/ZD-1952.pdf).

What I am getting on my output pin of the ZD192 is a very clear square wave signal which corresponds to the 800ms pulses I am sending from my transmitter. However, instead of being between 0 and 5V which is what I would like, they are from -3V to +2V.

Question: What sort of circuit do I need to change the bottom of the square wave from -3V to 0V (and the top of the square from +2V to +5V)? In other words, I need to somehow 'shift' the wave up by about 3V so that when there is no wave, I will get 0 volts.

 

[MikeMl]

Nothing on the spec sheet you posted suggests that the output pin would be between -3V to +2V. I'm guessing that you had your scope AC-coupled when you observed the waveform.

 

[MikeBrady]

I disagree. I can clearly observe the on/off behaviour of the receiver (via the output waveform) and it then flatlines when I break the beam from the transmitter. Looks good to me.

 

[MikeMl]

I disagree. I can clearly observe the on/off behaviour of the receiver (via the output waveform) and it then flatlines when I break the beam from the transmitter. Looks good to me.

I understand that you are detecting the beam ok. I was referring to why the output swing wasn't between Vdd (gnd) and near Vdd(plus supply), as the spec sheet clearly shows it.

 

[MikeBrady]

Ah, I see what you meant. Yes, you may be right. I'm pretty much a novice at using the scope - its one of those digital ones, with precious little doco. When I next get a chance to play with this, I'll see if I can find any settings related to the AC coupling. Thanks for the tip.

 

[eblc1388]

I agree to comment by MikeMl.

Disconnects the output of the receiver and test its output voltage again, using CRO.

It is possible the connection to the following circuit has (wrongly)imposed the mentioned voltage onto the receiver's output due to bad design.

 

[MikeBrady]

Thanks for the thought, but the circuit is unlikely to be incorrect. It consists of only 2 components
a) 5 volt power source from a high quality, ripple free DC power supply
b) the ZD1952 which only has 3 pins - Ground, Power and Output

The power supply is wired to the Ground and Power pins, while my measurements are being taken between Ground and the Output pin. Unfortunately I am away from home right now, so can't follow up with the advice to check the CRO, but that seems to be the most likely explanation.

 

[Dean Huster]

I think Mike M. is quite correct. He's pointed to the most likely problem, simple operator error. If the scope is AC coupled, it will cause exactly the behavior described, especially when you know that the circuit absolutely must output between zero and +5 because of the power supply considerations and data sheet.

The copling switch on most analog scopes is directly underneath the attenuator. However, digital scopes often make that a menu option where coupling can be selected (AC, GND or DC).

Dean