Do you get any thawab for teaching others about technology free of charge? Or is there both sin & thawab for both good use and bad?
The major points here are:
- your intention
- your sincerity
- and whether what you actually teach is halal or haram
I've addressed the two first points to some extent in my answer on Will I get rewarded if I study worldly sciences with intention to do good in future?
Now you should know that teaching might even be an ongoing charity (sadaqah jaariyah) صدقة جارية:
When a man dies, his acts come to an end, but three, recurring charity, or knowledge (by which people) benefit, or a pious son, who prays for him (for the deceased).
(Sahih Muslim, sunan an-Nasa'i, sunan abi Dawod, Jami' at-Tirmidhi)
Imam an-Nawawi has commented this hadith in his commentary on sahih Muslim as follows:
The scholars said: the meaning of this hadeeth is that the deeds of the deceased come to an end when he dies, and the renewal of reward ceases for him, except in these three cases, because he is the cause of them: his son is counted among his earnings, as is the knowledge that he leaves behind through teaching or writing, and ongoing charity, i.e., a waqf (Islamic endowment). End quote. (source islamqa #69948)
There's a similar hadith in sunan ibn Majah saying:
"The Messenger of Allah said: 'The rewards of the good deeds that will reach a believer after his death are: Knowledge which he taught and spread; a righteous son whom he leaves behind; a copy of the Qur'an that he leaves as a legacy; a mosque that he built; a house that he built for wayfarers; a canal that he dug; or charity that he gave during his lifetime when he was in good health. These deeds will reach him after his death.'" (sunan ibn Majah)
So basically if you are teaching for the sake of Allah and teaching things that are halal you will gain rewards.