# circuit with drive buffer

#### paardenvlees

##### Newbie level 4
Hi,

I build a circuit wit a drive buffer. When i make use of the logictoggle input from proteus the buffer(74HC365) is working correctly and when i replace them by just voltage sources and toggle the voltage source high low, it is not working anymore?

I have an output of 3.3V/0.0V from a comperator circuit and i want to use that as input to the OE. on the inputs at A1 there is also a 3.3V input.
Is this not working in proteus or what am i doing wrong?

#### KlausST

##### Super Moderator
Staff member
Hi,

I don't understand your textual explanation, nor do I understand what function you expect...

But as soon as /OE are high then the output is disabled.
Both /OE need to be LOW.

********
For a real circuit:
* you need a power supply. And the HIGH input level depends on the power supply voltage.
* no input should be left floating

Klaus

#### paardenvlees

##### Newbie level 4
i mean that if i simulate my circuit with logic gates on the inputs than the buffer works normal. So low, output at Y1 is high. and high output at Y1 is low.
If i do this the same with voltage sources on the inputs the component gives alaways a low output at Y1.

#### paardenvlees

##### Newbie level 4
Hi Stenzer,

the supply pins are correct, so this is not a problem. These are hidden but VCC is set to 5V. The component is working with the logic inputs from proteus and gives a 5V output at Y1.
See pictures below. The output with the supply voltages should also be 5V but it is 0V.

#### stenzer

##### Full Member level 4
Hi,

have you tried to set your supply voltage to 3.3 V? According to the TI datasheet [1], the minimum HIGH voltage level is 3.15 V @ 25 °C and V_cc = 4.5V. By having a look at the V_cc = 6 V entry, a higher HIGH level input voltage can be seen. Thus, you may expect a HIGH voltage input level larger than 3.3 V @ V_cc = 5 V.

greets

#### KlausST

##### Super Moderator
Staff member
Hi,

With 5V logic supply, then 3.3V input signal is not HIGH.

With CMOS logic (HC), valid high levels are 0.7V x Vcc = 0.7 x 5V = 3.5V.

--> don't use 3.3V but use 5.0V as HIGH.

And please understand that there are three output states:
* HIGH
* LOW
* Z = high impedance